also, i found that a lot of it has to do with the way the camera influences your facial expression. that smile for the camera just kills it for some people (incl me!

) experimenting and learning which poses work better for you helps a lot!
another thing is that you can only get certain angles in you mirror. whereas some pictures might be taken more from the side accentuating certain features...
try taking a handheld mirror plus a wall mirror and use it to look at your profile. you'd be surprised to see a different you!