I always found the best way to deal with artists is to give them a great deal of creative control over what the comic looks like. Give them a plot line, dialogue, and a basic character description and let them have control over the rest. If it looks well done, then tell them good job and try to get it published. Only nitpick when it something that has to do with the plot. (Well, So and So always wears gloves because it is a symbol of his emotional distance from society).
Artists, like writers, are very creative people. They probably have projects on the back burner that they would rather do then be a slave to your creative vision. If you let them have great creative control over the part of the comic they are responsible for creating then they will be more likely to stick around.
