If you looking for “natural” link building techniques (where other sites link to your site without you having to pay anyone), here are some techniques / ideas from Matt Cutts:
1. Write something controversial.
2. Create lists like top 10 ways to do something.
3. Give away something for free like software, ebooks, etc.
4. Participate in the community, comment on blog posts, forums, etc.
5. Create a resource that’s useful and even if you don’t have enough people linking to that page, it will still feature in the search results.