I performed an unusual backlinking campaign for a mid-sized NYC based software company. The result ended with the acquisition of dozens of PR5's and PR6's at a cost of approximately $1.50. Here's how:
I scouted potential site owners that would have interest in owning a complimentary copy of the company's software.
I contacted these site owners/webmasters and shipped free copies with an MSRP of $99 - $299 in exchange for a simple inbound link. Everyone was so enthusiastic about putting the links on their sites after receiving the free sofware. Many even put links and free banners on their homepages! Each copy sent cost the software company only $1.50!
I am currently applying the same model for a client who manufactures garments. The production and shipping costs are inexpensive, yet the products are potentially valuable to the right site owners.
Offering site owners and webmasters something other than a reciprocal link (many are bothered daily by these requests), may be a good idea for your backlinking campaign as well. If you are an e-commerce merchant, it may be a good idea to offer discounts or products at cost.
A final note: Donating products or services to non-profit organizations is a valuable technique as part of a quality backlinking campaign. In addition to the warmth and goodness you'll feel inside, many orgaizations will repay your kindness with a thank you and a link to your site. And non-profits usually have decent, good, or excellent PR.
I hope my tips offer you some insight on effective link building strategies.
Avi A. Wilensky