the best chalk for marking lines is called Miracle Chalk. It comes in a powder form, or a small bar of soap ( like you get in hotel/motels), or a stick. You only need to apply steam and it comes completely out. I actully pass my hot iron over it and it workds that way too. At this point it only comes in white and works great on dark fabrics. I emailed them awhile back and asked if they had a chalk for light fabrics. Thye were working on it but were not happy with the results they were getting and wouldn't release it untul they were 100% satisfied. And of course, here is a video on it:

http://www.youtube.com/watch?v=XfhfkVTvQ-0

There are other versions of the same thing available now but cant remember the names but have seen them often either in stores or various websites.