One useful idea that I began during the fall 2012 book fair was to take a library cart and fill it with one or two copies each of books for $5.00-$5.50 or less. This was very helpful when a younger student would walk in with just $5.00 and all I had to do was point them in one direction. It also allowed me to entice the older students into buying a book, which let them get a little school supply item for free.
Another idea that I tweaked from Scholastic was their candy guessing game. Instead of allowing everyone to take a chance and guess "How Many Readioactive Worms Are n the Bucket?" I only let students who purchased a book enter the contest. It doesn't really sound fair to students who don't have enough money to buy a book, but it did encourage several students to trade in a toy for a book. That made the restrictions worth it.
I also gave all students who purchased a book, a bookmark. I always have random collections of bookmarks to give away (mainly because I'm kinda stingy with the bookmarks at the beginning of the school year).
It never hurts to mention to students that they can buy a book and get an item from the school supply area for free. Usually, their book is the most expensive. Also, it doesn't seem to occur to some of them that they can buy the book in order to get the little school surprise for free.