Facial expressions are one of the hardest parts of modeling. Practice smiling with your eyes. Try parting your lips to show just a tiny bit of teeth. Try relaxing your face muscles or tensing them up. Try tilting your head to either side or tilting your chin up or down. See what works for you. What may seem like an imperceptible change in any of those things can make a much bigger difference than you think. One technique that some models use is to make very subtle vowel sounds with their lips. EEE.. OOO.. AHH.. it's a little silly, but it can really add a lot to a photo.
Another popular technique is "the squint." A slight squint of the eyes can create intensity. It also helps if you have a problem with your eyes looking too wide in photos, as if you're staring. Careful not to furrow your brow though. Don't scrunch up your whole face. Just your top and bottom eyelids to narrow your eyes.
Just to reiterate, the camera will pick up changes in your expression that may seem imperceptible to you. Changes to your body pose can be extreme and dramatic, but changes to your facial expression should be slight and subtle.