what kind of fabric are you using? I'm on the design stages for a wall handing using RK Kaufman's Radiance--it's got a nice sheen to it and I plan to do a whole cloth--but wondering about the marking just like you! And of course it's white so no white chalk....:( which is my go-to choice.

I think Bree has you covered with the best options for not marking directly on your fabric. Hope you find the best solution for you.

I use soap slivers all the time. An alternative to brushing the lines off, is to use a steam iron at a slight distance and the soap marks will disappear.

I've used tailor's chalk quite a bit. I like to vacuum it off when the marks are no longer needed.