For me stress was also a big problem. I scored consistently between 670 and 720 with the various practice CATs. The days before my actually test I got completely nervous. All I could think about was getting that 700+ score.
The night before the actual I was so nervous I decided to take another practice test that I had not taken before -- got a 720. Great. I thought. But for some reason that got me even more nervous. I was tired from all that stress yet I couldnt sleep that night. I was hot, sweating, and kept tossing and turning. Next morning I woke up at 4am and couldnt go back to bed. I was extremely tired but wanted to just get up. It was too cold out and so I decided to exercise on this silly machine we have at home. Took a shower and eat afterwards. Felt even more nervous then because I didnt sleep and I know how that would affect me. Then this is when things really gone down hill. I decided to go to the store and get a can of Redbull. I never had Redbull before but figured it will be better than to go in sleepy. I drank about half of it on the way to the test center and saved the other half for the break between AWA and Quant.
To my surprise, I did better than I thought in quant and failed miserbly in verb. In the end, it did not matter. I scored 570 -- the worst score I have ever gotten -- even lower than before I studied for the test. I am going to retake the test in a month.
Guys, what I have learned from this experience is you are suppose to take the real test while you are in the same mental and physical "modes" that you normally are when you study. If I had normally studied with a Redbull in one hand while only slept four hours. I probably would have done better. But I didn't. Another thing is just relax -- I know its easier said than done but like other posters suggested, it's important. Exercising definitely helps me. Even watching a DVD or something will get my mind off the test.
Also, leading up to the test I was over studying. I studied and practiced about 8 hours each day for 5 days leading up to the test -- which was ridculous. I have read that some people force themselves to not touch any GMAT material the day before the test and just relax. That makes a lot of sense to me and I will do that for my next test.
To sum it up, study but don't over study. Remind yourself the GMAT is only one component of the application process and its not the end of it. Also remind yourself that you are competent and capable. Constantly refer back to the practice test scores and see how they have improved overtime. That should give you comfort. Sing, dance, run, or do whatever that makes you relax and happy. And above all else, don't drink Redbull or anything other substance that can alter your thinking ability if you are not used to it.
Jeff