jxc,
Are you talking about simple affiliate programmes (i.e. links with reference numbers "hidden" in them) or actual contractual level "partnerships" ?
If just the simple variant, you'll normally find the average payout is very low, and if you look at their T&Cs, you'll find you'll need to accumulate a certain sum before they'll actually pay out.
What sort of products would you be looking at ? Have you done your homework and researched the competiton and how well they are doing ?
Personally, wearing my web surfer hat for a minute, I never use affiliate links or websites. When I discover a link to some other site with a book or something I might be interested in, I always clear down my browser (to make sure no trackers are left) and then go direct to make the purchase.