2. Make it discrete - What needs to be designed? The entire UI and workflows? The application icon? The internal navigation glyphs? Redesigning an entire app that has tens or hundreds of contributors can be daunting, but getting someone started with something small can be a gateway.
3. Leave it alone - One of the other commenters mentioned how a designer contributed something and then it was overwritten and ruined. Typically you don't want to have dozens of designers all working on a project unless there are fairly strict guidelines to work from. Have a working group at the beginning that can set the UI tone and create guidelines from that. Also a small review committee to review changes is smart, just as they do for code.
4. Version Control for Dummies - Put together a short explanation of how to use the version control system of choice, assuming no knowledge beyond Photoshop. Or find someone on the technical side who can be an intermediary/GIT trainer. The human part of this is very important.
Do those things and you should see prettier projects in the future. I know I'd love to get involved, but have no idea where to get started!