Before you head out on any trip, you'll want to be fully prepared. We're not going to re-invent the wheel & list all the things that anyone has ever thought of to bring with you.

I did find some good resources that you can use to make your own decisions:

We use the 4x4trails.net trail rating system

But we also like the trail descriptions found on traildamage.com

Are you a beginner 4x4 driver? Here are two good resources:
About.com Beginner Resources
Wikipedia Off-Road

And of course, a couple of checklists to get you started:
4x4 Trails Checklist
Offroaders.com