you can prep yourself for the format, but not the content.
the simple truth is that the test examines your overall knowledge among what is deemed to be your cohort, and therefore you can't suddenly learn a thousand vocab words because you sluffed off over the years in english class.
it's very simple. be rested and don't second guess yourself. you want to finish the test, so read a question, eliminate the obvious wrong answers, and then select the best answer. i didn't prep a lick for it and i completely killed that son of a bitch, but then again i'm the smartest person on the planet.
i thought the act test was more challenging than sat, and depending what colleges you're applying to, you may look into that test as an alternative. likewise, think of it this way... you really DON'T want to get a whizz bang score that's super high if it doesn't represent your natural level. you want the test to be accurate in assessing your levels. why? say you luck out and get a super high score that's really not a good barometer of your levels, you will go to a school that relies on this base level and if it's not you, trust me, you WILL fail. the test is to examine you, so let it do its work.