This is an all-to-common problem that many freelance artists encounter. If you're being paid a flat rate for something (meaning the whole project is one price like $300 and that's it) you MUST have a few sentences in your contract about "changes" to the artwork if the client wants you to redo something. I usually put something like "Each change to the artwork will be charged on a per hour basis of $25 per hour. A change is defined as something the client changes their mind about, not a mistake by the artist." This protects the artist from doing 2 or 3 times the amount of work through endless changes. There are differences between minor and major changes too, which can vary (like changing a character's hair color vs redrawing the entire page layout). Since I've put this in my contract, I haven't had any problems :)