Most of the people I know who have made significant money through affiliate programs were involved in something shady like pornography or weight loss supplements. Like a good SaaS business, they can get $50 a month from you for years; however there is all sorts of trouble you can get into. Some of those guys are pretty fat and they show up at Vegas for a trade show and somebody asks them how well the weight loss supplement works for him and he says "I don't know, I never tried it!"
Some people have also made vertical-oriented or horizontal-oriented affiliate sites around the likes of Amazon.com and they usually get a few years of the biggest paychecks they ever ever seen but then they start having good months and bad months and they don't like buffering for that so they move to Seattle and go work for Amazon.com.
Whatever type of affiliate marketing you want to do you should make a realistic plan about how much traffic you can get at what cost, what kind of conversion rate you might get, commissions you get, etc. That is the most important thing about any partner you might choose.
A good one you could consider is SkimLinks?