Relax!
Look where the sun is (if u are shooting outdoors) in relation to where your 'group' is.
Shoot RAW (do you get to PP or do u just hand them a card?)
Assert yourself so you get people in the right positions, simple stuff - tall people at the back, shorties up the front.
Use your histogram, it's your friend.
Make it as fun as possible. Be motivated and enthusiastic, that goes a long way.
If something doesn't go right, move on...
PS - U thanked me a little early
Oh yeah, don't cross your fingers, even on a Canon it will make using the dials and shutter somewhat difficult
Good luck, let us know how you get on.