The key to battling test-day jitters is thorough preparation! But full preparation means more than knowing the test material. It's important to prepare your mind and body for the experience of working under pressure.
Get some Sleep:I know you've heard this before, but it's often repeated because it's so important. Get plenty of sleep the night before the test! If you don't have enough rest, you'll find it much harder to concentrate.
Leave Early. Be sure to leave earlier than normal on the day of a test to avoid the added fear of arriving late. You don't need the extra worry of heavy traffic.
Eat something--even if you're nervous. Next, be sure your tummy is happy! If you're too nervous to eat before you leave the house, grab a banana or a bagel and have it ready in case your stomach growls just before you enter the building. Hunger is distracting in itself, but the added embarrassment of a rumbling stomach will break your concentration. You might also find time for a quick bite during a break.
Take a Quiet Moment. Once you arrive at school or at the testing center, find a quiet space where you can be all alone. You can use your car if you drive, or you can find an empty locker room or office (ask a teacher).
Once you locate a quiet place, do a relaxation session. Close your eyes and try to relax completely. Take deep breaths and think calming thoughts.