alienfluid,
Here's some advice from a GMAT expert that I found on the net. Hope it will be of some help.
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Day before: no more than 60-90 minutes of high-level review. Relax that evening, have a good dinner, get a good night's sleep - all the obvious stuff.
The day of, about 30-60 minutes before the test, do some VERY easy practice problems in your area of strength (math or verbal). Just do some from the low numbers of the OG - and DON'T look at the answers (don't even bring them with you). You're just warming up your brain here, not trying to learn anything.
And plan something to do afterward, no matter what you get - meet friends or family for a drink or food. Just acknowledge that you accomplished something, even if you don't get the score you want that first time out.