Use the Certified Arborists' Study Guide. Much of what is in the book is outdated, but nonetheless, that's what the majority of the test will be based on.
Assuming that you're already a member of ISA, log onto the member's section and look at the CEU quizzes they have there. Your test questions will be in similar format.
I recommend paying particular attention to the trees growing in nearby proximity to the test site, as they are most likely to be on the Tree ID test. When I took mine, I had to travel across the state, so I popped for a hotel room near the test site and spent the afternoon prior to the test becoming familiar with species growing near the site.
If you can buy or borrow the CD version of the Arborists' Study Guide, it's great to listen to while you're driving around in the truck. When you get to the point that you can recite the narrative along with the CD, you're going to be pretty well bulletproof.
Best of Luck!