Focus on quality links and quality traffic - PR increases automatically, but is really just an aftereffect that is not used by search engines to rank your site.
Posting on related forums with a good signature also helps quite a bit. It takes time however, but then again, everything takes time.
This is one of the best analogies I've seen yet.PR is like an extra straw in a McDonalds Bag. You get excited for a second and say "Hey, they gave me an extra straw!" but after a few seconds that fades as really it's of little to no use to you!