Hey Athenacus,
I'm sorry that your test anxiety affects you so much. The way I tried to keep myself calm about my exams is in the way I study. When I've gone through everything and think I've studied everything, sometimes I would panic because I felt like I knew absolutely nothing. So I'd go through and imagine several questions that I could see being on the test and I'd answer them to myself, without looking at anything. That was one way that I used to reassure myself that I really knew the material, which then helped me from getting so nervous.
Also, I tend to remember where things are on the pages of my notes. So, if I'm sitting in the test and I just cannot think of an answer, I'd try to remember where I had it in my notes and picture that spot on the page and see if I could then remember what was there.
Just in general, though...try to take it one question at a time. You will absolutely panic if you think about the whole test and what happens if I get a C or if I blank on the whole test, etc. etc. Concentrate on the question right in front of you. Don't second guess yourself - I've changed way too many answers because I second guess myself (and I always ended up switching away from the right answer).
I think the important thing, though, is where that anxiety is coming from. Is it from a fear of failure? If so, what kind of failure are you afraid of?