Networking and getting to know other people in your niche i think is the key. If you can form a network with other vendors, and all of you have quality products, you can all promote each others work. I guess if you are not huge or don't have a big reputation yourself already then you need to find affiliates who are not "super-affiliates" but maybe just "good" ones. Get friendly with one or two of them, have them share the deal with the rest of their personal network of contacts and you'd be surprised what can happen from there. At the very very beginning you may have to beg a little to find someone who can really help you get your foot in the door.
I get approached by vendors directly through my website, we get approached perhaps 3 to 5 times a week right now to affiliate with various vendors. I personally do not pick who i work with solely based on the affiliate commission, but i definitely know a lot of affiliates who do. Quality of the product is a huge issue also - if you have a reptuation in a niche you want to keep it - the last thing you want to end up doing as an affiliate is to market something which totally sucks, it can have a knock on effect on affiliate income for a while afterwards as your buyers may lose trust. (i'm speaking from experience!)
The main big problem with networking of course is that it is easy for you to get screwed over by someone if you happen to hook up with a nasty person. So you have to also have spades and spades of intuition and investigative skills - aboslutely knowing your niche and the products in that niche inside and out will really help with this so you don't get suckered into something by a shark.