My favorite marking pencil is a graphite aquarelle pencil in medium, sharpened to a fine point. Aquarelle means washable.
The graphite lead shows up on light OR dark fabrics, stays for much longer than chalks or pouncers, and, although it can rub off over time with the movement of your hand over it there is often still a faint line.
It helps to have the light you are woking under come more from the side than shining directly above your work.
This pencil *does* wash out, though it may take a couple of washings if you've marked a hard, thick line.
Google "graphite aquarelle pencil" and you'll find many brands and sources.
Jan in VA