First a quality product that people will actually want. Then I look at gravity. The sales page should also be well laid out and well written.
(1) A product that makes sense to market to my target traffic to my current site. (2) Doesn't look spammy(long damn sales page with massive headers and cheap copy writing) - looks professional. (3) High commissions(30%+ or $15 per sale and up, commissions like $40, $50 dollars are most favorable)