Being in sales I have always learned that if you take good care of your clients they will take care of you. I have turned people away because my product was not right for them and even told them what they should be looking at buying and those people have sent me prospects. Of course I am a likeable guy and people want to do business with people they like. I truly care about my clients not just as clients but people. We will talk about vacations, sports, kids, whatever. I even track their birthdays. That all might sound cheesy but it works. When I ask my clients to refer me to other people they do it knowing I am not going to screw them and take good care of them and if my product is not going to work for them then I won't sell it to them. I don't waste their time and I won't waste mine. The best referral system is a straight forward one, just ask!
Of course you could offer $50.00 for every referral that buys too.