Do practice problems, see overviews of the test's contents, and even take full, timed practice tests with accompanying scores. Note: you must make a (free) account with Khan Academy in order to use this resource.
Generate worksheets focused on just about any math area you find challenging to hone your skills.
Exercise your knowledge of grammar (especially those comma rules that they love to ask you about on the SAT and the ACT!) with these practice questions. Then, browse the rest of the website for explanations about nearly any grammar question you can imagine.