If you want artists to work on your games, how about bringing them in as equal partners from day 1?
I've realized this from day 1 with my own project. No one wants to work on someone else's dream game. If you're asking them to work for free, the least you could do is ensure them a seat at the table for determining the game's general design (not just art design). Here's the exact words I put in my most recent help wanted ad seeking artists:
This project has six active team members at the moment. We want to continue building a team of dedicated, passionate individuals who share the vision of what Allacrost is to become and want to be a part of making it happen. Becoming a member of this team means more than simply getting told what to create. We strongly encourage people to participate in design discussions and offer their own ideas to improve the game and the project itself. Although the core design of Allacrost is pretty well-defined at this point, there's still a lot of unanswered design questions and features we have not yet implemented. There's still plenty of room for you to influence this title should you be interested in doing so. While we are seeking core team members, we're also happy to welcome contributors who prefer to help out here and there with adding a new feature or creating new art or music, but aren't as invested in this project.
And this is further reinforced on our new contributors page, which is the very first thing someone that is new or interested in joining the project would see. I think it's a good set of policies that our team has, and have helped this free project survived where others have failed. Despite all this, it doesn't seem to be enough to interest artists any more than your average "come make art for my awesome game" type post.
Earlier in this thread someone mentioned that because art can be instantly evaluated without needing it in a game, artists aren't that interested in contributing to a free project because they can build their "portfolio" without needing any game project at all. This is true and a very good point. But then shouldn't the same hold true for composers? I can listen to a piece of music and feel how talented the composer is. They don't need to contribute to game projects, yet I get so many interested that I have to turn some away (we really can't have more than 2-3 composers realistically).