Open sourcing a project, releasing an existing project, under an open source license, is hard to do well.  The SnapLogic blog has a good essay on it, Constructing the Bazaar: Taking advantage of the open-source development model in your project.  The three main points in the article are:

1. Modularity
2. APIs
3. Remember the guy that doesn’t have time to come to all your meetings or check out all the modules.

All those will make it easy for someone to contribute to your project.  I'd add, make it cool. 

• Don't open source a completely baked project.  (Is there such a thing?) 
• Leave some cool parts and features for the other guys. 
• Along the same lines, be open to new ideas and new directions for your project - otherwise you'll always be saying no to the guy that wants to help out.

Good luck!