Big things:
1. Minimize your caffeine intake prior to the test.
2. Don't eat a big meal before the test.
3. Get a good night's sleep
4. Do things that improve your mood and state of mind the night before, whether it's watching a favorite (innocent) movie -- I'd stay away from spy thrillers and mysteries
5. Don't look at the polygrapher as an enemy -- s/he has likely heard all kinds of garbage so isn't going to get excited when you admit to taking pens or paper from work or post-dating a check or performance review