@Zydalc
TheMeadTree, nailed the point in ths kind of situations.
The best advice to make easier getting an artist (or anyone actually) to work on something for free (specially on something that involves so much work and time investment) is to make them want to.
How to do that? Showing them something amazing that will get them interested on being part of it. Show them that their work and time won't be wasted, that there is passion involved on the project, that is something serious, and that it will remain active until its completion. Most artists will fear that a fangame gets shut down or abandoned after someone in the team lost interest or gets interested on something else, making all their work go to waste.
So if you are a programmer have a small but nice demo to show of the features you are going to have in the game, if you are a composer have a sample of your music that people can hear, if you are an artist have a portfolio to show how good is your art, and so on.