To encourage your students to participate in a class review (or other comparable activity), hold a raffle at the end. For every correct answer the student supplies, she earns a raffle ticket. The more correct answers, the more tickets, the better chance to win. If you're using this for a test review, the most coveted prize would be some extra points on the test. Otherwise, use something that works into your pre-existing system.
Raffle tickets are available at office supply stores or online. Or you could create your own.
