Study the fundamentals, dude. The ACT is all about what you know!
Get “Getting into the ACT” (the official stuff) and practice as much as you can. I would also try “Cracking the ACT” (from PR, which IMHO is the next best thing after official stuff)) for general strategies and for more practice tests. I have the 2005 book (I think 2006 is out now, not sure, don’t need it! lol) and it is really solid.
What are you scoring now? What are you trying to get to?