One of the hard things about any type of rewards program is that you can earn the most really fast out of the gate and then it starts to trickle down because not as much new stuff comes out. Plus, you usually get a sign on bonus. So your first cashout is generally pretty easy. The good news is that most programs will not disable your account for being inactive for awhile. So you can back off and then go back a couple of months later and see what is new.
Try to look for survey opportunities. Those tend to renew more than the freebie and signup type things because new surveys are always being added to the various programs. And surveys tend to be the lowest risk, most legitimate way to earn money anyway. I have some sites I do (like NFO/Mysurvey) where I may not do any surveys for 3-4 months and then can all of a sudden go in and do a bunch really fast to get a check.
Also maybe try something similar but a different route. Like BzzAgent. You don't earn "rewards" per se, but you get really good coupons and sample items. And they add new things each month.
Hope that helps.