Simply an unending process of finding what works for you. You need a product or service that sells and searchers who're looking for that product or service. You're job is to be a medium that connects the two. Success feeds on itself, so once you find a combination that shows results, stick with it and keep developing and refining what you're doing. Don't expect instant success (don't quit your day job), but be prepared to spend at least a couple of years (or more) doing a LOT of learning before you MIGHT be able to approach doing affiliate marketing as a living, rather than just as a sideline.