I'm another artist-programmer, so I'm possibly not the best person to ask for advice. But on the other hand, I've also been an artist on other people's projects that I've slipped off of and generally lost all motivation for, as well. I would say:
Have a prototype. Use placer art. DO NOT POST until you get some good stuff to show.
Also, have a design document. Well detailed, not Kilometric. Just something everyone can read and say "Oh I get 100% what it is about"
These are both very important things, for two reasons.
Firstly, as has been mentioned before, artists don't usually understand code and also have little idea which programmers are the good ones and which are the bad - a playable demo without art, which is nearly always possible, can show them that your game is worth helping out with, it's a cool thing that they can make even more cool with their artistic efforts.
Secondly, you don't just want an artist who turns up one day and says "yeah, I'd like to do game art", you want an artist who will actually stick with your project and not abandon you after ten weeks. The more cool you convince him your project will be, the more likely he is to stick around... and conversely, the less of your project you explain to him up-front the more likely he is to discover something about it he doesn't like later and decide it's not something he wants to be involved in. Not to mention that it's a buzz to see something you've drawn moving about and spitting fire (or whatever) in-game, which helps keep the motivation levels high; you want to be able to do a quick turnaround between the artist giving you art and that art appearing in the game, to help keep them interested. Ideally it would be instant, they can just save over your placeholder art in the data directory and it's there.
Lastly, I'd also add that it's also worth being open to discussing the rest of the project with your artist, not just giving him sprites to churn out. This is part of the "an artist is a team member, not a commodity" attitude, I guess, but really - make sure they feel like you're listening to their opinions and allowing them to help make a cool thing rather than enslaving them in pixel-spriting bondage.