but hasnt google advised against selling links. So if you buy a link from a site, eventually this site will be found out and all pr and backlinks from this site will be as good as nothing, yet you will have still paid for it!
I would say the best way is to use natural methods through forums, blogs, and related directories that offer quality content and are rated decently. You want to imply statistical measuring through out your entire effort of selecting sites of which to put forth marketing effort. Focus these efforts on websites that are higher in statistical value, and you will see a much larger "Bang for your Buck"