I'm guessing you've only modeled a few times? Usually after 4-5 shoots you start to get a lot more comfortable. I can see some nervousness in your facial expressions, and they lack any true expression, but I've found that to be pretty normal for new models.
My best advise would be to start shooting as much as you can. Don't just shoot with anyone though, make sure their work is at least halfway good
. Facial expressions are always the hardest thing to master.