The Vertical Measures team is serious about the practice of link building--so much so, that they've devoted an entire blog to it. This post focuses on developing a balanced link-building campaign, which includes targeting directories with real editors (people that ensure the content stays updated), as well as forums, which the team calls "the virtual water cooler." It's also important to find a few partners for reciprocal links, and to use social media for contextual links from blogs and reviews.
But a balanced link building campaign isn't just about variety, it's about consistently being a source of fresh, relevant content. Link building is an ongoing process--there's no point in starting off with a bang, saturating your sources with content and hoping that the results continue indefinitely. You have to make a long-term commitment to developing content and actively promoting it if you want to stay ahead of the pack.