Not sound engineered for a gig in years, but from memory:
Band listens to what they're being told (near impossibility from previous experience);
Band does what they're told, when they're told (again, virtually unheard of);
Dont play over your band mates individual line check - wait your turn;
While you're waiting for your level check, stand where the audience will be and give some feedback to the SE / musician;
When sound check is finished, DO NOT change levels / EQ on pedals / amp etc. (guitarists I'm particularly looking at you!) ;
Buy SE pint - he/she can make you sound awful (worse than you can even manage yourself!).