Like Tartan, I use regular school chalk which I sharpen with a pencil sharpener. After I mark it, I spray it lightly with some cheap aerosol hairspray to temporarily set it. Both the hairspray and the chalk have washed out completely for me. Surprisingly, this includes one that I had marked with blue chalk on a white fabric that sat in the UFO pile for many years that I thought would be stained.
I also agree with Tartan about the tailor's chalk--I used to use it for garments, and remember scrubbing more than one item with Dawn and a toothbrush trying to get the marks out.
FYI, I save the powdered chalk shavings to use in my pouncer.