Know what are the "hot buys" nowadays and for sure many will be motivated and will be interested to get the prize at stake. For example, at DotComSecrets, they have given away a Hummer (won by Mike Filsaime), iPhones/iPods, exotic vacations, rolex and many more.
Giving prizes is also a great way to show appreciation. Have some fun with it, and in turn people will be excited to participate. Personalize this idea for your company and the response will be phenomenal!
Motivating your affiliates is something you need to work on to. If you want to put your business on the next level, then you need to bring out the best in them.