Hey Dude
I know exactly how you feel like, I got mines on the 8th of May so I've been revising loads but this is some of the advice that I've been told.
For the short passages on the critical reading you should quickly have a look at the questions first so you actually know what your looking for. Also at the top there might be a small introduction about the passage, READ THAT because that helps a lot. When your stuck between two answer ask yourself why this would be right.
On the sentence completions dont look at the choices straight away but put your own word in so you know what type of word your looking. Also you can tell a lot about a word by just saying it out loud, like if its positive or negative.
Make sure you know the structure on how to write the essay because its not really hard, you just have to know what to write for example, you should have 5 paragraphs, 3 detailed examples, use advanced vocab where you can that you do understand and make sure you use both pages.
Get a vocab list and a little notebook, and just write down random words you hear anywhere, and start reading newspaper like the economist and all the other stuff.
When it comes to grammar it just comes down to practice, and learning the rules that collegeboard tests you on, you can find these on sparknotes.com and also some really comprehensive iinformation on the critical reading parts of and good Flash Cards
Good luck and if all fails try the ACT
What colleges are you thinking about applying to ?