When you look at a pair of lips, what is your desired retouched outcome?

The desired outcomes are as different as the starting images.

That said, l do have a small lip "cheat" folder that I can drag to beauty shot
for a small headstart, it's actually just blank layers, that have blend modes
and layer styles baked into it, so when I paint or clone to it, it's half done
and all that I do is just tweak the color and size of the inner glow and a few
other things and done.

What is it that you're looking to do with lips in an image?

I use a very small, soft edged "paintbrush" for almost everything. I also put one or both lips on a separate layer, and use the up, and down keys, with the move tool selected, to "add fullness". The final result MUST look absolutely natural - I DO have ideals, though. Being an artist, and not concerned with the original look of the model, this technique is not good for everyone, but can be very useful, if done skillfully, even for them. What I look for is the lips, (or any other part), to match the rest of the image for sharpness, color, and all other characteristics.
-Don

Im not totally certain what your question is...but it seems like I think you are asking... what do we think about as we are retouching lips and lipstick?

Here are some of thoughts and concepts that cross my mind when I begin to work on the mouth and lipstick and gloss etc.  Its a bit long unfortunately...because there is a lot of detail and elements to consider in a lip retouch.  uugh.

For me it depends on the model, the image, and intended end look.

Some models are very soft, and very natural, and very little makeup is used in the original image...for them... sometimes I find myself not wanting a sharp, hard lip line...but one that matches the soft lines and edges elsewhere.  And then ill let the lipstick kind of diffuse into the existing skin...its that soft, natural, transparent look.  These kind of soft natural models use a large diameter tube of lipstick that is quickly put on without a lot of precision.  It is kind of rubbed on or scattered on haphazardly. This is almost like a lipstick wash... instead of the perfectly applied beauty lipstick.   Sometimes a more nude or translucent color and shade is used.  On this kind of retouch I would think about using some of the translucent blending modes like soft light, overlay etc.  I would not have a defined line on the top or bottom lip. I would let the color diffuse from the lip line onto the skin because the large diameter tube is not very accurate and it is wide and a bit sloppy on the edges.  Thats for the soft, natural, nude look.  I would use a clean, clear, curve based adjustment layer to retouch with... with the red channel maybe pulled up a little bit to create a slight redish color...and the rgb channels pulled down slightly to give me a darker translucent see through red color.  I  would paint with a very soft white brush to not create any hard edges.  I might even use a multi-spot stipple brush to further spread and diffuse and blend the edges more softly.

Other models that might be fashion or editorial models... i might be very interested in matching the colors of the dress or something in the background with the lip color...so it looks so coordinated and thoughtful!!  LOL.  If its a classy or polished original image I might also add some of my lipstick gloss brushes to the lip...to give it that classy expensive sparkle. In fashion type images...the clothes are the main focus of the image...not the model, or face, or lips...so lipstick would not ever be overly strong or draw undue attention to itself.  For other fashion looks...the lipstick is almost a part of the look itself... and there it can be a bit more saturated or contrasty to make it stand out or pop out.  You can use the HSL and Contrast Adjustment Layers to create this color saturation boost and contrast pop (using the associated black layer mask).   Your original image and the purpose of the image will give you the direction to go in... and by how much.  Most all lipstick creation in the magazine world is done with curve based adjustment layers because they are totally clean and clear, and they can be made any color, and they blend in perfectly with the underlying lip textures and glosses, and the effects totally adjustable in perfect positioning and intensity.

On higher end beauty models... sometimes the image is very tightly cropped around the face...( A headshot  or portrait) and the model may be making a fashion or makeup statement with her looks.  If she has perfect foundation and eye liner and thick eyelashes.. it may clue me in that I might have to treat the lips as high precision also to match the overall high class look.  High end mouths for me can consist of:   perfectly pencil drawn edges to the lips, a precisely drawn and sculpted cupids bow with gorgeous curves and a nice dip, lipstick edges that follow the contour of the mouth perfectly...and then beautifully colored and filled in lips... with possibly some nice gloss and texture enhancement.  There is a lot of work and Detail that a MUA puts into a gorgeous hollywood or NYC lip.

If the MUA has made any attempts to use a lip pencil to draw the outline first and then fill in...then that is the clue for me to make sure that the edges I make are as sharp and hard and perfectly drawn in for my retouch work.  I will be sure to make any lines that are soft or smudged or incomplete become perfectly drawn in and completed.  Ill pick up the color of the outlining lipstick... and work near pixel level to make sure I get the line hardness and curvature and direction really perfect.  This is often done with the clone stamp tool at medium opacity on a separate transparent layer. The cupids bow is a critical area to make it just right with symmetry and matched curves and just the perfect amount of dip.  There may be a bit of lip gloss shine I will add from my shine brush set or from other highlight areas.  Sometimes the cupids bow highlight extends up to the central part of the nose...I want to be sure I match that highlight intensity, softness and line so it looks even and balanced.  Some MUA's also put foundation on the lips first...so you will have a perfect lip pencil outline and also perfect skin next to it as well.  If the skin is not perfect... I will make extra effort with the clone brush to make the lipstick line perfect...and then... the skin next to it perfect too.  I will often clone in the right tone and texture of skin that is necessary to get that effect.  The right brightness, color and texture of skin may come from nearby rather than directly across from the area you are working on.   That way I get a perfect penciled in lipstick line that looks perfectly applied... and the foundation that abuts right up against it also looks bright and perfect as well. 

Sometimes the original lipstick will be sloppily, or messily, or quickly applied by the model possibly... and she will not perfectly fill in the the side to side corners.  Depending on the end product... I will certainly seek to FIX the application so it looks professionally applied to the entire lip area evenly.  Maybe sometimes messy lipstick might be a part of the tone or message of the image...in that case ill make sure my retouching matches the character and intent of the story of the image.  Some people like casually applied lipstick and others like to use a precision brush and application.  You will want to stay consistent with the look of each.  I will use the clone brush at very high magnification to make a precision intended line look totally perfect... and I may use a 5 single pixel dot random stipple brush to get a smudged look where required.  Sometimes a model will inadvertently eat something and smear her edges unfortunately...then I have to work on the edges and the abutting skin as well.

Sometimes ill get a high end image that is classy and glossy...maybe a beauty or jewelry image.  There I will try to make the lipstick application as professional as the rest of the image.  Mostly it will be a sharp, tightly drawn edge and filled in.  There I will try to make a perfect drawn lip even if it doesn't exist because the intent of the image is to be classy and perfect.  If there is any specularity in any other area of the image...then I might take that as a clue to make sure there is a nice lip gloss also applied.  I have a set of lip gloss brushes that apply just the gloss itself ...and I can manipulate the gloss to fit and look photographically perfect.  You can make your own brushes like this because it is terribly hard to hand draw believable lip gloss textures on the lips...getting it to look right and natural and sexy is hard to do by hand...so borrowing the gloss from another area or image is the perfect way to get a realistic gloss pattern.  There is nothing more luscious and sexy and high class than great lip gloss.  I try to use it where ever it warrants.  Something else shiny in the image gives you the clue that its OK to use a nice gloss in this image a lot of times...but be certain if you possibly can to ask the customer what they "specifically" want "in writing" if you possibly can.

I will often work BOTH sides of a lip line with the clone brush tool and work them against each other to get the perfect line of foundation and lipstick edge in the perfect line and shape.  Ill especially work in the cupids bow, on the edge of the side of the lips where the lip itself becomes diffuse and indistinct.  If its a high end image ill clone out the side of the mouth and insert a pure line of lipstick clean clear to the edges for that Perfect LOOK...if its too perfect and obvious...I might use the layers opacity slider to blend it back in so it looks clean... but not totally perfect for the right balance.  I use a separate layer for each different effect...so I can control and change it easier later if needed...and I usually try to label the layer so I know what it was used for. 

Often there will be a roll on the lower lip that produces a shadow under the lower lip.  Shadows and blur love to hide under the lower center of the lip and it sometimes gets kind of dark in there too.  So I will clone over that area with a lighter color and texture of skin and then opacity blend it to begin to remove the shadow and dirty look.  I might also have to pick up some other skin texture and move it into the area because the clone look looks too smooth.  I have some skin pore textures that I use that fill in these smooth areas with natural looking skin pore textures...then you cant tell that I was smoothing in that area... because the texture hides the softness and makes it look real again.  The lips, the penciled in edge line,  and the abutting skin are all a part of the perfect retouch.  I will usually work on all 3.

On the lip area itself...in order to look 3d and have some depth to it...I will usually slightly darken the top and bottom of the lip and then lighten up some of the middle center part of the lip.  This gives the lip some three dimensional depth and shading.  Makes it fuller and more realistic because of the shading...just like in applying eye shadow colors.

It is also sometimes necessary to add in some dark texture vertical lines (fissure lines) in the lip area or especially at the edge of the line into the skin area.  Sometimes lips can have a serrated kind of edge texture to them...and if those crevasses are eliminated...then the line and lip doesn't look real.  These crevasse textures can also be pulled from another image and reused to rebuild the dark, deep vertical texture fissure lines in the lip itself... and also at the very edges.  You can also clone some existing fissures into other areas of the mouth...or use another image and transfer them over.  Or create some brushes for yourself to make the work quicker.  Some models have deep crevasses...others have slighter less deep textures... or even smooth lips.  You will want to be aware of this and match the natural character of the models lips.  Use a black layer mask on that texture layer and use the white brush to reveal the textures you want...and how strong you want them. White brush to reveal them, black brush to conceal them...using a black mask as the base.

Most all professional lipstick color is produced by using curve based adjustment layers.  ANY color lipstick color can be produced and the application is always clear, clean, transparent and no underlying lip textures are ever covered over or destroyed... unlike using the regular destructive Photoshop painting methodology.  Sometimes blending modes can be used to further change the tonality of the lipstick colors.  A red color can look hot red...or if a multiply blending mode is used... then it will become a beautiful dark maroon...or if a screen blending mode is used...it can turn that hot red lipstick into a hot pink lipstick color. Go ahead and experiment.  So for lipstick creation...you are always looking to create curves for the color and the base tonality... then using blending modes to further enhance how the application blends into the underlying lip... and also always using the opacity slider to control the saturation or the depth of the color itself.  So,by using all 4 channels of curves, and by using blending modes, and the opacity slider almost any color and look of lipstick can be created from hot red, to total crimson, to hot pink, to blue, or gold, or fucia or orange or gothic.  Because the method of application is non-destructive...the lipsticks color, depth, and brightness or darkness can always be changed instantly and interactively on demand at any time...even after you have saved the image previously.

These are some of the thoughts I might have on applying and retouching lipstick.  Sorry for the length...getting beautiful lips have a lot of aspects to them.  These ideas are not written in stone... you can do a lot of things in Photoshop different ways sometimes.  I do like this non-destructive approach however...because it allows me to quickly create any color or shade, to work completely clean and transparently, to never covers things up, and it has the ability to almost instantly alter or change any aspect of the application... without any quality loss or time loss.

Have fun...experiment...knock em dead.