You basically have two options. There are fairly expensive marker pens sold for this express purpose by quilt shops. In theory, they will stand the test of being used on a quilt, although in practice you get a few bad reports. Option #2 is to use a marker pen which is designed for children and is sold as being washable, including from fabric. These are a lot cheaper and getting fairly popular, although there isn't as much data with these as there is with the ones sold by quilt shops. Again, most people like them but you do get the odd horror story. I've just started playing around with the Crayola washable markers myself.