After reading the posts on cost of marking pens and chalk marks not coming out of the fabric, I got to thinking about teachers. They use chalk everyday, invariably wipe off their hands on their clothes, and that chalk washes out.
On my current piece - I'm at the hand quilting stage - I marked the motif I wanted to use with white, 99 cents a box chalkboard chalk on the dark blue and red fabric. Quilted, and to my delight (and relief) the marks came right off. Used both methods - dusting it off and also water wiping it off.
Worked like a charm! And I have plenty left for drawing on the sidewalk!