It's weird staring in to the big, gaping maw of a huge camera. And unfortunately, that weirdness can appear in photos. This is where friends come in handy.
If you bring a friend with you, there's someone to relieve the pressure, make you laugh, and possibly distract you from the menacing click click click of the camera.
If you're a bride, bring your maid-of-honor. If you're a high school senior, bring your BFF. If you're a mommy to be, bring along daddy-to-be or a good girlfriend.
Whoever your buddy is for the day, make sure he/she is someone who can comfort you. Someone who doesn't make you nervous and self conscious. You'll thank them when you see your beaming face in pictures.