At the beginning of a test, quickly write down key facts, formulas, etc. That way, when you need the information to solve a problem you can find it easily, and you will save time and make fewer errors.
Answer the easiest questions first, then work on the ones that have a lot of points. Show your work, and try to at least get a start on all questions—partial credit can add up a lot of points.
Try to answer every question, or at least get a start on it. And NEVER spend all your time on one question!
Everybody makes mistakes in their work. But, mistakes can make you end up with a wrong answer. In math, a calculation error can give you the wrong answer even if you use the right method! So go over your work and check for errors before you turn it in.