I read this hint in the $100,000 quilt challenge magazine recently and have been tinkering with Crayola classic thin washable markers. I am still a bit nervous, but so far so good. It draws on my quilt easily without tugging and having to apply too much pressure. When done I just wet down a "car wash towel" (in other words a washcloth used for chore work vs. my face) and it comes right off. I use a marker that resembles the print color area closest (just in case) and it works great.
Now that it is back to school time, the markers are pretty inexpensive.
Two rules though...use a soft touch with your lines and DO NOT expose to heat.
Before discovering the markers I usually use thin slivers of ivory soap or chalk. cheap, easy and washes right out.