What is the best way to incentive different types of affiliates?

We run a freemium model with Riipen as there is no pay wall for companies to sign up. This has made it difficult to find the right incentive model to motivate different types or personas of affiliates. Motivation and retention seem to change as time passes, depending on the kind of affiliates. Have you seen higher levels of motivation and engagement when the incentive model changes, for example, offer bonuses or giveaways to top-performing affiliates? We have offered the same fixed incentive model for 3 of our affiliate groups, except for our default group, to prevent fraudulent activity. It would be interesting to see how other companies can manage fraud and suspicious activity while monitoring a tight incentive program. I would be curious to hear your feedback/thoughts.