I've tried a variety of fabric markers including AlanH's recommended plain-old-chalkboard-chalk. I can't keep a piece of chalk sharp, can't get close enough to the straightedge, and the sound! Well, you know where "fingernails on a chalkboard" came from. Makes my whole body break out in goosebumps.

Ran across the following gizzie in the quilting section of a fabric/craft store. It holds a quantity of powdered white chalk (refills available) and doles it out through a tiny toothed wheel -- like a very thin bicycle sprocket -- that revolves as you run the marker down the length of a straightedge. The picture shows it upside-down, the wheel is on the rounded end and the chalk powder loads in along the straight edge (with a plastic cover to keep it inside).

The chalk is a bit loose, and will shed excess, but the "bottom layer" goes well into the fabric weave and stays until washed out. You don't need to go back-and-forth, that just piles up excess powder that will migrate around -- one stroke does the trick.

Cost about $3.50 IIRC and refills were equally reasonable. If you can use AlanH's standard chalk, more power to you, but this is my newest favorite tool.