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:)
I'll tell you the biggest challenge...tracking through to the end of the funnel. Many affiliate sources are unwilling to provide this access which creates two problems, 1) trust in the numbers and 2) the inability to know what method is working best.
I'd be happy to share my best practices for getting around some of these issues.