Basically what i do is i base my niche on a niche that has lots of forums.
Then what i do is i contact the owner of these forums and ask them if i can advertise on there site.
Now i know that this has been done but i ask them to do it in a different way.
What i do is i ask the admin to post himself in his forums telling his forum people to buy the product because it is great.
This works out great every time because the people trust the person telling them to buy the product and its very cheap usually much less then putting ads on there site and you convert much better i would say.
For me 2 days ago a admin posted for me on his forum for 5 dollars and since them 17 people from that forum have signed up.