This past weekend I went to a quilt show in Modesto, CA. While there, I bought a marking pen, made by Pilot, called FriXion ball. The marks are removed by friction; either by rubbing with the end of the pen, or by ironing. I watched a demonstration and it was amazing. I intend to give it a try on my next quilt project.
ETA: These pens are available from various office supply stores, Amazon, etc., in several colors. And no, I don't own any stock in the company. :mrgreen: