To piggy-back on the other excellent advice here a few things:
1) Show up in plenty of time so you can
2) Look good (the way you're dressed -- these are "free" points you should never give away) and
3) Feel good (stretch or do whatever you need so your body isn't an issue getting to your spots).
4) Focus on one (or two, if you're an overachiever) thing to really emphasize for yourself each game, or even quarter, if you're subbing in and out.
5) If you played or know basketball, like someone earlier said, just go with it and
6) Make your calls distinct and forceful. You blew the whistle, so now you have to tell EVERYONE what's going on. There's no taking it back. (That includes speaking so your partner knows where to go next -- Remember: He/she was watching his/her PCA, and not yours.) Think of yourself as a traffic cop, not just a "judge" out there, and that will eventually frame your movements better.
7) A corollary to No. 5: Just get the call right. If your mechanic is clumsy or late or downright wrong, that's secondary Besides, if you do it confidently -- which you should -- you might look more like a veteran out there. You know, the kind who aspires to do more college (or NBA) ball, or who has watched too much TV, or just figures after XX years, the people will know what they're gesturing about.
