My first recommendation is to determine whether you have an option of taking either the SAT or ACT. If the school will accept either test, then you need to determine which test is more suited to you, and take that one! This article has a comparison of the two tests, including a matrix you can use to help evaluate which is the better test for you! It also has free downloadable study guides for both tests.
Sometimes people overlook the easy stuff. By this, I mean those things that require no studying at all. This includes getting enough sleep, eating a good breakfast, being well-hydrated, and exercising. You need your body to be in top condition to improve test performance.
I have one suggestion for multiple choice tests. Let me demonstrate how it works. Suppose I gave you a ten question true/false test, in Japanese, and you do not know Japanese. If you randomly selected true or false for each answer, you could miss all of them. However, if you put either all true, or all false, you would likely get about 50% of them correct! So, what I do, is walk into the test with a letter in my head. Today, I pick "B." For any question that I flat out do not know and cannot solve, I will put B, and move on. But, what if I can narrow it down to A, C, or D? or A or D? simple . I will take the letter to the furthest left. Mark "A", and move on. This frees up a lot of time and reduces stress. It is NOT a substitute for studying! Also, if I run out of time before finishing a section, I will mark "B" for all remaining questions prior to time running out. A few of those will be correct!
Also, I recommend you find out how the schools you are applying to will use the scores. Do they use only the total? Do they also use one or more section scores? Which score do you need to raise - only the overall score, or, do you need higher scores on a particular section? If your only focus is the overall score, it may work better if you study material you are already strong in. That is, if you are strong in vocabulary, it may be easier to improve the vocabulary score than to try to get to be good in math! I took this approach on a promotional exam - mastering my strengths, rather than improving my weaknesses. It worked really well - I outscored everyone else by 20 points! This is not to say you should ignore the other areas -just focus more on your strengths!
Hopefully these suggestions help - best of luck!