Most libraries around here measure minutes read, instead of books. This is more fair to the older kids who read longer chapter books. It works really, really well. Their goal is 10 hours in six weeks. They get little prizes every 2 hours, plus they get raffle tickets everytime they come to a program or come in to fill out their record. The raffle prizes are the really cool stuff.
We tried rewarding based on how often they came into the library last year, because our director wanted us to. Pretty much everyone hated it, so we're back to minutes.
75% of the kids will finish one record and about 40% will finish two. We're doing a grand prize for those that read at least 20 hours this year in the hopes that we'll get more to read 20 hours.