Ze Frank has started a blog of notes and advice about fostering online participation. Lots of good stuff so far.
Usually there will be a few contributions that are outliers in technical merit and scale. There is a temptation to reward these contributions by drawing specific attention to them while the project is running. This can sometimes have the effect of damping the project as a whole, since potential contributors will measure their work against an artificially high standard. Alternatively, only displaying the most recent contribution allows the tonality of the project to be at the whim of the last contributor.
Instead of only focusing on technical ability, draw attention to qualities that can be expressed by anyone: simplicity, individuality, and humanity. Allow there to be a feeling of “Hey, I could do that too”.