Hardly ever make money on the ones that offer a flat $150 or so for "qualified" affiliates.
I did make a fair bit with Extremepaychecks with that method though. Not sure how many sales they had to send to be qualified. I probably would have made more if it was percentage based.
Here's a couple off hand that give me a steady check on referrals alone:
PussyCash
AEBN
These two have two-tiered referrals, which are nice:
AdultDateLink
DatingGold
In my opinion, percentage based is the way to go.