Hey Diana, I'm no expert but here are some of the ways I have coped with stress and the GMAT:
1. When doing practice tests (and for that matter, on the real test) convince yourself that the scores don't matter. I like to think that the real test is like any other practice test and I can always do the GMAT again. Once I started doing this and stopped worrying about scores, my performance improved. The less 'special' I make the GMAT event, the less nervous I am about it.
2. Along the same lines, refrain from thinking too much about the GMAT 'scoring algorithm' (that is, of course if you do). I see many folks on this forum trying to correlate their raw scores to the number of questions they got right or wrong. Focus on doing your best and leave the 'judging' to the GMAC peeps.
3. Try and laugh about the questions that you get wrong. During study or while reviewing wrong answers on a practice test, I always laugh about the questions I get wrong (I get a mental picture of Homer Simpson going 'doh') The point is to acknowledge your 'silliness' as to why you got something wrong, figure out why, learn from it and move on. This mental attitude will be helpful on test day when you won't have a choice but to move on.
4. This may or may not be helpful: Try and keep an ongoing mental commentary in your head when doing problems. This may sound freaky but talking to myself (usually in my head but sometimes low level muttering) actually helps me maintain a problem solving frame of mind. I find this especially helpful on data sufficiency and reading comprehension. My mental commentary is like discussing a question with a friend except I'm discussing the problem with myself.
I hope that some of this stuff is at least some what helpful to you. Good luck!