At the foundation: Affiliate marketing can be termed as marketing a product or service better than the merchant itself. But what I have seen (from the Affiliate Management side) on what distinguishes high performing affiliates from lower ones: They show a different side of the product or service, incoporate it into a larger service offering, and/or push for real value to their audience. That, in turn, builds a loyal audience base because of their unique or specialized position on these products or services.
My suggestion, take a look at Zac Johnson: http://zacjohnson.com/ He has some great tips; from starting out picking a niche to scaling your current, successful affiliate marketing tactics.
Good luck! If there is anything else I can help with, I am only a call away:)