Most important... get the 'myspace Face' out of the way early!
You know.. the one with the lips in a kissing pose.. the one they practiced in the mirror for hours right before they came to the shoot.
example
I usually let them do it for about 10 minutes or so just to get it out of their system.
During this time, I usually just make a shutter sound with my mouth to limit wear and tear on the camera.
Once they're done (their lips usually get tired or cramped).. they have to smile to get their face back into human form... And I am prepared... I rattle off 8 frames a second!