Take a stand whenever possible, and, if you can honestly take the OPPOSITE side of what you think most test-takers will choose and write a coherent argument, do so! AP Test Scorers thrive on reading something different.
If something in current events immediately comes to mind, AVOID using it. Chances are, if it's popular and it came to you that quickly, it will be used by a lot of test-takers. AP Test Scorers will be sick of reading the same thing.
Stay away from The Scarlet Letter if possible--sometimes, it just works, but MANY test-takers use it.
Use readings with which you are familiar but that are relatively unique.
Remember, your audience consists of English teachers who will be more familiar with literature, so stay away from your music, soaps, TV shows, movies, etc. (in general) as sources. If these are used, have them be supplemental, not central.
Sometimes, qualifying the argument is almost a must.
Use your nonfiction book, if possible and use some of the less usual books you've read. This gives the Test Scorer something different and refreshes them (as long as you effectively back up what you use).
Stay away from religion! There's a reason religion has been debated for thousands of years... it's unsolvable and unprovable. Find something with less divisiveness.