Just adding my $.02...the tests are important, but not for great reasons...if a school gets 10,000 applications, which ones do they read, and which ones do they just dump in the trash? They'll look at the applications with the highest test scores, and work with those...basically, they are a weed out tool.
I haven't kept up with the research, but I'll assume that the old findings hold, which is that the best single predictor of freshman year grades are the SATs or ACTs. (Guess what the single best predictor of sophomore year grades are? drum roll...freshman year grades!).
In a nutshell, the college you are applying to has no idea what a 4.0 at one high school means to a 4.78 at another high school...the tests thus provide a "standardized" way to compare one applicant to another.
Now, having said that, you can likely influence your score, simply by being familiar with the test question formats, and practicing completing the test questions while observing a time limit. As pointed out above, the old analogy questions ("A is to B, as C is to ?") would throw people whon had never seen them before. My 10th grade teacher had us practice them, and once you did a couple, you got the point (and then could do them rapidly on the test day, and thus get a higher score).
However, I would highly recommend not to bother cramming for the test...you won't remember all that stuff you learn at the last minute. Your time would better be spent, in my opinion, doing several practice tests, learning about the types of questions, and then doing some relaxation exercises so you are ready to perform your best on the day it counts.
And if you think the ACTs suck, get ready...if you want to go to grad school, you will have to take a grad school test, like the GREs, GMATs, or MCATs...and likely there will be other tests as well. Looking back, my scores were fairly high, and helped me get into a good college as well as getting a scholarship, so again, I wouldn't say they aren't important, but they are not the only factor.
But for right now, the best way to eat that elephant is one bite at a time...good luck!