This is a question community managers hear a lot. Everyone wants to grow their project and we all want to hear what worked (or didn't) for others. One of the benefits I have by working for Red Hat is being part of a large community of community managers and community oriented people. Stormy Peters put this question to that group last year:
We are all working to grow the communities around our projects, in particular diverse communities (diversity of company, gender, geography, etc.). What's the single best tip you've gotten for attracting new contributors? Who gave it to you? Or, alternatively, what's the single thing that's made the most impact on attracting new contributors to your project?
She got a lot of great answers, some of which I've collected over at the Red Hat Community Blog where this was originally posted.