To echo previous posters, I agree that test anxiety can be a major impediment to doing well on the GRE. I feel compelled to respond to this thread because I like SO many people, have struggled with test anxiety. It has gotten so bad that I just accept that I won't be sleeping before a big test, interview, etc. Yes, I know, it can feel overwhelming at times, but I recognize that ultimately, that's the real source of the problem: It's not the anxiety itself, it's how we mishandle that anxiety so that we actually inflate our anxiety and make it worse.
If you repeatedly underperform on the GRE, as it appears the original poster has, this just exacerbates the problem as well. In Malcom Gladwell's new book he outlines this phenomenon of "choking" on standardized tests. He writes that "after repeatedly failing on these standardized tests, [...] the usual prescription for failure- to work harder and take the test more seriously" actually makes the problem worse. You are just adding to the anxiety and pressure! Like a mouse running on those wheels. Instead of working harder, it might be prudent to rethink our preparation strategies and work smarter. Perhaps we shouldn't be studying more, or purchasing another GRE prep book, or memorizing more formulas. Perhaps what needs to be addressed is how we approach the test and our anxiety associated with it.
When we are extremely anxious, we tend to be overly cautious and apt to second-guess ourselves (see Metta's comment above). Malcom Gladwell writes that carefulness and second-guessing are "not the way to succeed on a standardized test. The more you do that, the more you will get away from the intuitions that help you, the quick processing." If you are trying not to screw up on the test, you're putting more pressure on yourself, and thus inflicting more anxiety. How you approach the test is so critical: try framing the GRE experience as an opportunity to demonstrate your outstanding abilities!
I think the posts have been spot on; one of the most effective ways to prepare for the GRE, when test anxiety is an issue, it to immerse yourself in it! A little systematic desensitization in action. This means that you need to take a health dose of "practice" tests. I put "practice" in quotes because you shouldn't really approach them as if they were practice. You need to simulate the testing environment: So don't take the diagnostic GRE test while you are laying on the couch in your pajamas and easing Cheetos.
Be serious about them, but at the same time, when test day rolls around, let it go: laugh a little, see some friends, get your mind off the GRE, and just go in to the testing site and take it. Sounds simple right?
Develop some perspective on the matter: yes, the GRE is important, but it's just a test. However you perform, everything will be OK. The world won't come to an abrupt halt.
Another thing that I have found is that if you fight your test anxiety, and view it as something that you need to get rid of, then it grows stronger. When anxiety rears its head, accept it. Allow it to be, and recognize that it doesn't have to control you and how you perform on the test.