Run a fair program. Treat your affiliates with respect. Do everything you can to make sure you convert as well as possible for your affiliates. Be visible. That doesn't mean you have to pay for promos but it does mean selling your program and doing things such as including it in your sig.
Good promo material is a must these days. Most affiliates rely on your promo material. I can't tell you how many niche sites I would promote but can't because the promo material is lacking. Make sure you have material for tubes, blogs, TGP, MGP and a variety of types (not just FLV format!).
One thing works against word of mouth from webmasters and that's often that we want to keep the best converting sites a secret for ourselves. But run a fair program, be visible, and provide good promo materials and chances are everyone will eventually find you and a give you a shot.
