On a dead ball when a player goes into hostile territory (the opponent's bench) Go In and get him out.
Never turn your back on the play. You will find on a pass right at you that you'll want to "get spun" away to officiate over your shoulder. Try to spin towards the play.
If you're going to be a BJ for a while and aren't going to have on-field play clocks invest in a Ready-Ref timer or something similar. They are a lot better than taking your eyes off of the play to check a wristwatch.
The Original Ready Ref Football Timer
Keep your distance
Never backpedal with a whistle in your mouth.
Never backpedal into a goal post.
Watch crossing routes for "pick" plays.
Watch WR on potential crack-back blocks.
Never go on the field with just one bean bag. (Its tough to bag both first touching and end of kick with only 1)
Goal Line!
When you drop back with a punt receiver maker sure he know what a legal fair-catch signal is.
And as has been said already, be able to quote rule 6 like its your wedding vows.