Make up a sheet with all the formulas you need to know and memorize all the formulas on the sheet.
When you get your exam, write down all the key formulas on the margin of your paper so if you forget them when you're in the middle of the test you can look back at the formula.
Read the directions carefully and don't forget to answer all parts of the question.
Make estimates for your answers... i.e. if you are asked to answer 48 x 12 = ?, you could expect a number around 500, but if you end up with an answer around 5000, you'll know you did something wrong.
When figuring an algebraic expression, do one step at a time and bring the rest of the problem down.
Check over your test after you are done with it. If you have time redo the problem on a separate piece of paper and see if you come up with the same answer the second time around. Look for careless mistakes such as making sure the decimal is in the right place, that you read the directions correctly, that you copied the numbers correctly, that you put a negative sign if it is needed, that your arithmetic is correct and so on.