How do you make people think, "Gee, I want to contribute to this project?" That's the first step they have to take before they can become contributors. Well, traditional wisdom states that people contribute to open-source projects because:
They like helping
They enjoy being part of a community
They want to give back
They think that something is wrong and they need/want to fix it
So you may want to make it apparent that help is needed, that an enjoyable community is there, that giving back is appropriate and appreciated, and that there are problems that need solving.
Now, to be fair, this is an area that I don't have fully mapped out or figured out for the Bugzilla Project, yet. So I don't have a lot of personal experience to draw on. But if we analyze other projects, we can see that some good ways of getting contributors…