Can you use the bathroom during the test? For some reason I get nervous and always have to get up and use the restroom during a problem set. Any insight?

You are officially only permitted to leave the room on the breaks. If an emergency occurs during the test, you can raise your hand and explain the issue to the test administrator. S/he may allow you to go to the restroom, BUT:

1) the test will not stop - your time will continue to tick away while you're gone, and

2) the administrator will document the fact that you left the room while the test was going on - if you leave repeatedly or for a long period of time, the information will be reported to GMAC and your test could be cancelled as a result

So, what to do? Don't drink anything within a couple of hours of the start of the test. Don't drink caffeine (it's a diuretic). Do make sure you're hydrated enough - you don't want to be dehydrated in there - but try to keep your liquid consumption low so that you're better able to control this psychological urge.