Sign up to some CPA network and pick an offer that is related to the niche your site is... Then come up with something that will get them to sign up.
ex. if its an iphone website with all iphone related people, then you could send them an email saying "I just got a free iphone, and so can you! just go here (<- link to cpa offer) and sign up!"
You might have to do a better job than that to get them to sign up. Anyways, good luck!