Many affiliate programs set it up so that a higher revenue share is paid to the "super" affiliates. Basically, this means that affiliates who sell more are rewarded with a bigger percentage of each sale. This can be set up with multiple tiers to reward each progressive step up the chain with a higher percentage. This can also be done for specific time periods to encourage affiliates to make a big push and move inventory that needs to be moved.
*When you select affiliate management software make sure it is flexible enough to handle these set-ups if you think you may want a tiered solution at some point.