Approach is everything with a thing like that. I would consider calling it a partnership rather than you are looking to have them as your affiliate. First, compliment their website, then ask if they have advertising options available. Then mention you have a revenue sharing partnership available that is working well in a few instances. Another approach is offering to do a "hybrid deal" whereby you pay them a certain monthly fee, plus giving them a commission per sale (which can be lower than what you offer straight affiliates). This could be a good way to show some good faith like you are trying to make them some guaranteed money plus the affiliate commissions.