For me, the true referrals always come from happy clients. Happy clients usually will always refer no customers and purchase additional service.
We tried doing the affiliate program thing. What it came down fto is the affiliates did not sell anything because they were just sticking banner ads up. The customers who were already referring didn't want anything to do with the affiliate program. So, the only person who was making money was the affilaite service provider.
I would prefer to throw a bill credit on to a clients account every once awhile as a thank you for being a good client then you have just made us meet sales quotas for the month.