I think that 'not testing well' can be something of a crutch that some students use to rationalize being OK with getting a bad grade. Rather than falling into that trap yourself, try to understand exactly why you are not doing well and getting problems wrong.
For example, are you nervous because you're not as prepared as you could be? Or overconfident only to be rudely awakened by problems that you dont have a clue how to solve? Or just sloppy, and make lots of arithmetic mistakes on problems that you know how to solve but still get wrong? Do you budget your time poorly and spend too much time on one problem at the beginning of the exam only to run out of time and start rushing, making mistakes as you near the end? Once you figure out why you're making the mistakes and getting the problems wrong, that will tell you what you need to work on, what to practice on in order to get better.
Here are some tips that have worked for me.
1) At the start of the test, skim through the entire test first. Get a sense of which problems are easy and which are hard, and which are worth lots of points and which are worth only a few.
2) Skip around the test and do all of the easy problems first and get them out of the way. (Just make sure you record your answers in the right places on the answer sheet!). If some easy problems are worth more points than others, do those first. Rack up as many points as you can, as early in the test as possible. This helps to take some of the pressure off when you start to work the more difficult problems.
3) When you've selected a problem to work, read the problem slowly and carefully to understand exactly what the problem is asking for. This is particularly true of word problems. Then, figure out a strategy for solving the problem.
4) Budget your time. Don't spend too long on any one problem, at least at first. And plan to spend more time on problems that are worth more points, less time on those that aren't worth many points, especially if they are difficult ones.
5) Check your results. If you solve an equation for x, say.. plug the value back into the equation to see if it really is a solution. This helps to guard against arithmetic errors.
6) Don't leave the test early. If you complete the exam with time remaining, go back and keep checking the problems until the time is up. Use all of the time available. You don't ever want to get a test back with points off for wrong answer that you could have corrected except that you left the test early.
7) Practice - do lots of problems in the days before the test. Do all the problems at the end of every chapter that the test will cover. This can be a lot of work, but how badly do you want an A? Check your answers and keep working the ones you got wrong until you understand why. Ask for lots of help. Just like in sports, the practices should be much harder than the games if you expect to win. This preparation is very important to doing well in the exam.
8) Get plenty of rest the night before.
Hope this helps, and good luck!
-Guru