Here's how the exam is set up:
35% Political institutions, behavior, and public policy
25% Social developments
10% Economic developments
15% Cultural, intellectual developments
15% Diplomacy and internal relations
About one-third of the questions on the exam will cover the events occurring in 1500-1789, and the remaining two-thirds covers events from 1790-1877.
Buy a book specifically for the U.S. History I test. In my experience, I found the REA guide on this subject to be phenomenal; it was very thorough, with three practice tests in the back and full explanations for every one of the questions on the practice tests.
Take the first practice test "cold" to see how you do overall and to see where you are weak. Now is the time to carefully and attentively read the book all the way through, going back when you're done to work on the rough spots you noticed in your first test.
Go to collegeboard.com/clep for free online study resources for the U.S. History I. There's a lot of good stuff there, including the link to history lectures given to AP students to help them pass their CLEPs.
Now take the other two practice CLEPs in your book, carefully reviewing any wrong answers.
You're ready to take your test! It will be administered on a computer unless you request otherwise, and you will be given a brief tutorial just before the test to familiarize yourself with the software.
One little test strategy to use on your CLEP: as of July 2001, no points will be deducted for wrong answers. Therefore, if you get a question that you don't know the answer to, give it an educated guess. Educated guessing will definitely improve your score.
By following these suggestions, you're on your way to making the best score possible on your U.S. History I CLEP exam.