I wrote several days ago about when to take the SAT or ACT. In that post I mentioned that students need to adequately prepare before taking an exam. But what is adequate preparation?
I generally recommend that students take a practice SAT and a practice ACT in the fall of their junior year. You should take the practice SAT test from the collegeboard and the practice test from the ACT. You can take the practice test at home but it should be under similar conditions to the actual test. In other words, work straight through the test, time each test and do not allow any distractions. After taking the two tests, determine on which you have the higher score and focus your test prep efforts on that test. To compare the test scores you can use this SAT/ACT concordance table.
Once you have determined which test to take then you need to decide if you want to prepare for the exam yourself or with the help of a test prep company. If you decide to prepare by yourself, then you will need to take as many practice tests as you are able. If you are focusing on the SAT I generally recommend The Official SAT Study Guide. If your focus is on the ACT I would recommend The ACT Prep Guide. The reason I recommend these study guides is that they have actual past tests from the companies giving the test.
As you take these practice tests pay particular attention to those questions you get wrong. If you can understand why you got a question wrong, it is less likely that you will get a similar type of question wrong later. If you follow these suggestions you have a great chance of doing your best on the standardized tests.