Uwhusky,
I am sorry to hear that you are having nightmares! Unfortunately, I still have nightmares about the GMAT, and I teach the dang thing.
Anyways, exercise is definitely one of the better ways to relax before the exam. Really, anything that takes a lot of energy is good. The basic idea is that you want to be so tired that you fall asleep naturally without "the fear" setting in before nodding off.
Go for a long walk at the local park/botanical gardens. Go bowling with some friends. Go out to eat, or go see a movie. The main thing is to NOT just stay home and sulk. (Also, do not get drunk. One drink to calm nerves is fine, but keep it to one drink!)
Also, this may seem silly, but I also have a "happy place" to go to when I get nervous just before or during the test. My personal happy place is an image of carebears running around in a rainbow forest. It is incredibly stupid (and embarrassing) but it makes me laugh every time, and therefore calms me down.
It doesn't hurt to give yourself a pep talk. I had a student who created a "test-day" mix-tape. It had songs like "Eye of the Tiger" and so forth. This all sounds dumb, but the problem is a psychological one, so there really isn't a specific solution akin to solving a wicked DS problem.
Let me know afterwards if any of this works for you, or if you find something else to calm your nerves. As an instructor for The Princeton Review, I am always trying to give good advice of this nature to my students, so anything helps!
Oliver Pope
GMAT Trainer
The Princeton Review