I use a white artists pencil called Swan Stabilo All Stabilo. It looks exactly like a regular pencil and leaves a straight line like a regular pencil. It is water soluble, and comes out completely. Unlike chalk it will not rub off until you wash your quilt (which I always like to do, since I think it makes them cozier) If you don't wash your quilts it will come off with a damp (wetish) cloth.
You can buy one at any art store or any place with a good selection of art supplies. It costs about a dollar.