My daughter just took the SAT, so I'll focus my answer based on that test.
Several months before test date, take a practice test. Use the results as a guide to focus your studying.
You can spend as much or as little as you want on test prep classes, of course. But I think that if you put in the effort and time, just studying from a test-prep book (they are available at libraries) can help raise your score. Read challenging material to increase your vocabulary (also something you should be doing anyway).
The days before the test: get good sleep. Exercise (well you should be doing that anyway, of course). This may be obvious, but eat your normal foods in the days leading up to the test.
Know what you need to take to the test (pencils, calculator, etc.). Have all that ready the day before, so you will sleep well knowing it's all taken care off.
Just relax, and good luck!