Love your point about the importance of a long term relationship - this is exactly right, I want a long term relationship with my partners rather than just a one off mail.

I think if you approach people with this in mind it will show in your email and they will see you are not just pushing them the same email as 1000s of other people will receive from you, but that you do want to work with THEM specifically.

This also filters down into writing your opening pitch to them with a little unique info - to show youve been on their site and that you know about their business / product and that they really are a relevant partner.

This personal, long term approach worked for me in our last launch, and I have kept in touch with all of my partners and will be working with them again too.