At the last one, I paid particular attention to the paperback books that were in good condition. I bought quite a few that I wasn't necessarily interested in keeping and then listed them on Paperback Swap. That way, I could swap those for books I had beed wanting to find.
We try to make it to each sale and usually come home with armloads of books for very little money. This is a great frugal way to build up your home library.
Be sure to go to Biblical Womanhood to see more frugal tips.