http://www.ananova.com/the-rules-for-backlinking/

Backlinking. It’s a touchy subject. One that confuses a lot of would-be bloggers and cheap hosting site owners. Should you do it? How often should you do it? Are there any real guidelines that Google has set on the topic? It all depends on how you look at it. Here’s a guide to blog backlinking that makes things (relatively) simple.