whats the best way to give really straight thick hair volume?

back brush baby back brush.

is there a certain product that should be used?

Like deadly design mentioned, back combing is great. you could use a little finishing spray if you must, but it will be easier to brush out (depending on how extreme the back combing is) with less product. a silicone based shine spray is great for brushing out the back comb, remember to start brushing at the ends and work your way up. large paddle bushes are great for that.

if back combing or teasing is not a option for you, i would suggest layers. if your hair is very long, and all one length, or close to one length, you probably don't have very much height in the hair making it appear to be flat. bringing your layers up will release some of the wieght, giving you more body, height, and movement. just becareful, take your layers up too high and you'll have a mullet. (unless you want a mullet)

Another point:
Set hair with huge heat rollers, spraying first with a spray gel, or hairspray and cool.

Then as the other two stylists said, backbrush.

Use a comb that has different sizes of teeth (looks like the measurments on a ruler). If this is for you, turn your head upside down and hold a piece of hair about 2" wide, start at the root and move out on the strand of hair. Spray hair while upside down and let it dry, then turn your head right side and smooth.

If it is for a client... do the same but don't make them turn their heads upside down!

Clair great portfolio! but now i know what you mean about your hair. If i were your stylist, i would say you would look fabulose with side swept bangs and long layers! My suggestion, get a new hair cut. you can leave the length as long as it is, but take the layers up, especially around the crown of you head. but don't forget, you can have a great hair cut, but styling is half the battle.

tell your hair dresser you want it blown out with LOTS of volume. then ASK QUESTIONS! what products she is using, why she is using them, and how she is using them. I always go through step by step what i'm using, why and how, so my clients can go home and recreate the style. i know that many hairdressers forget to do that because they may be running behind, or rushed, etc. so ASK ASK ASK!

Best wishes,
  Emily Chen