The two things that have kicked butt for me are loads and loads of social bookmarking, and writing link bait content. Social bookmarking is easy when you use OnlyWire or something similar. And link bait is just about writing articles that are kind of opinionated that you know will set people off on a debate. Oh and the third thing I've found works great is getting your link on the footer of a free WordPress theme. I got a surprise when one of my sites I'd put zero effort into suddenly showed up at PR3 just from a theme backlink alone.