Many students have trouble taking exams, feeling anxious and "blanking out" on what they know instead of being able to show what they know on the exam. Luckily, test-taking is a skill that you can learn to improve. If you experience this problem, you might want to take a look at this excellent guide to understanding and fighting test anxiety. The guide is written for people taking the Praxis exam (an exam students take to become teachers), but the tips are applicable to any testing situation and may give you some insights into what test anxiety is all about and how you can fight it.
Do you have any other good tips on dealing with test anxiety?