The best advice I can give is to do lots of past exam papers. But apart from that, try to take your time. If you're calm and relaxed about each question you'll probably cut down on the number of silly mistakes, simply because you're not rushing madly through the paper.
Sure ... taking more time during a test will reduce the amount of time at the end of the test that you have to check through your work, but I reckon that by that time, you'll probably not be all that interested in critically reviewing your own work.
Think about allocating time to each question. The recommendations are 30 minutes for multiple choice and then 20 minutes per question. You might have made up your own to allow time at the end or something like that. Stick to it. Chances are you may not need the whole time for the question, especially the early ones, so take your time and make sure you get the easy ones right. Use a bit of time at the end of each question to check your work before you move on to the next question.
And finally ... be confident! At least you know that your main problem is silly mistakes! At least you know your work and you don't have large slabs of work that you need to learn!
Good luck ... and do lots of exam papers!
