"Dig your wells before you get thirsty". This is an important principle to apply to all forms of marketing, but especially those that involve relationships, such as affiliate marketing. You should always make friends who could potentially be your affiliates before you even consider launching a product or service.
Create an accompanying "abridged" version of your product. This could be a limited piece of software or a book that is only 10 pages long, rather than the normal 150. Embed an affiliate link in these abridged product versions and offer them to other sellers as a sales page bonus. Allow them to brand the affiliate link to give them an incentive to use it.
If applicable, offer to personalize your product for each of your affiliates. For instance, if you are selling software, offer to create a special download after for them personally and even add text that uses their specific business name and information, rather than your own. This is always an attractive offer for affiliates who are interested in branding.
If you contact someone and ask them to become an affiliate for you, always remember to give them a free copy of your product or service. Not only will this increase their likelihood of actually becoming your affiliate, but it will also make them considerably more effective as an affiliate, since they will actually understand what the product provides.
Contact members of your list and offer to give them some special offer if they participate as affiliates. Keep in mind that they have a vested interest in your niche, which means they more than likely are friends with other people who do, too. Give them a special deal, such as a higher than normal commission.
Providing useful marketing materials is one of the most important parts of affiliate marketing. If your affiliates do not have good pre-made advertisements, including solo ads, classified ads, and pay per click advertisements, they may not be able to effectively market your product, especially if they haven't purchased it.
Make it extremely easy for your biggest partners to participate. If a potential partner will have to spend 10 hours in order to make money with your offer and only 30 minutes with another there is a good chance he will simply go with that other product. You would more than likely do the same exact thing.