Your prep should include deep analysis of what types of questions you’re missing and then practice of those, particularly in your “lower” sections. I find the ACT Black Book to be very helpful in this kind of analysis. It’s a companion to the Red Book, so you need to have them both.
That said, I have a question for you: why do you want to raise your 34? If you’re thinking a 36 will make you more competitive for college admissions, as others have mentioned, it won’t. Even those top 20 name brand colleges don’t make a distinction between 34, 35 and 36. You will not be more of an automatic admit anywhere, simply because you have a 36 instead of a 34.
However, spending several months prepping unnecessarily can give the colleges a bad impression, particularly at those name brand schools. A student who only cares about his numbers—grades and test scores—and not his development as a whole person is not going to impress an Ivy-type school. As listed on Big Future.org, Harvard cares about,
“Secondary school record most important; character, creative ability in some discipline or activity, leadership, liveliness of mind, demonstrated stamina and ability to carry out demanding college program, and strong sense of social responsibility important.”
Do you see what’s not mentioned there? Standardized test scores. In fact, if you follow that link to Harvard’s entry on Big Future, you’ll see the whole list of what they consider in your application. “Standardized test scores” is fourth from the bottom.
You will have a much better admissions process if you spend this summer focusing on how you can make your world, community or school a better place, instead of raising your ACT score a meaningless point or two.