I haven't done or organized any contests yet, but I believe that contests that does not require Affiliates to spend any cash or any other virtual cash such as (MRPs or TCredits) would attract Affiliates more. For example; (Signing in new ECAs, or attract new clients to TripleClicks Auctions and PriceBenders). So, if Affiliates, or some of them were successful to get such prospects, the'll be gaining VPs and cash.
I myself recommend suchh contests.