Took the GMAT last year for the 2nd time and did far better this time around. Some tips...
- Take the test as early in the morning as possible. The absolute worst thing I did the first time was take it after work....I was already mentally drained from a day at work, then walked into the test center for a 4-hour session. Bad move. Even if you're not a morning person, you'll be far less tired if the test is the first thing you have to think about during the day (well, presuming you don't go get hammered the night before).
- Simulate the test environment as much as possible. Take the computerized practice exams. The GMAT lasts 4 hrs, and I did a few 5-hr sessions. It's just a matter of getting used to how the test will play out. The first time, I'd study for an hour here and there, but never went the full four. It makes a huge difference in the end.
- At some point tonight, just stop studying. If you don't know it by now, you're probably not going to learn a whole lot of new crap and may just end up confusing yourself. Just go see a movie, watch some porn, whatever...and clear your mind for tomorrow.