Maintain the 12-15 yard distance between you and the QB. When he drops back, so do you, otherwise he'll make a sudden turn and you'll be in the middle of the play. (this is my toughest challenge)
Box-in - keep the play surrounded by officials. You have a wing to one side, umpire on the other side, back judge in front - so make sure you stay in a position behind the play to complete the box.
Did someone say slow down? Well, slow down some more.
When giving signals, pretend you're on TV. Slow (no, slower), verbalize your call just like you're on TV, that will force you to slow down - plus it forces you to think of the enforcement.
Don't stare! Keep your head on a swivel and your eyes moving. From that position, it's easy to get tunnel vision.