The Official Guide has sample essays near the back of the book.
Also in the OG, there's a complete list of all the topics in the question pool. It's a waste of time to go through them all very carefully,but you can get a very good idea of what you're up against.
I would strongly recommend writing an essay or two under the 30-minute time limit just for practice.
I wrote more about the AWA here:
http://www.gmathacks.com/gmat-awa/all-a ... t-awa.html
There's a section called "How to Prepare" near the bottom of the article.