I use the Sewline fabric pencils. They're ceramic 'leads' in a mechanical pencil and the lines come out by either erasing with any soft eraser or with water (I use a small artist's brush to just 'paint' water over the lines). The leads come in white, gray, and several colors for use on any color of fabric. I like that the lines are very fine, don't rub off while you're working, yet come out easily when you want them to.
http://www.sewline-product.com/Sewline_Fabric_Pencil