The % from each affiliate each time they earn something probably would be the best option if it doesn't get too OP. For example: User2 uses User1's affiliate and User1 receives 10% of User2's income. How ever if User3 uses User2's affiliate, User2 gets 10% and User1 gets 9% since he's the one who invited User2. And so on... Obviously Steemit would have to come up with some kimd of formula, but you get the drill.