Pounce pad. I've seen them at JoAnn's and quilting stores.
http://www.amazon.com/Quilt-Pounce-P.../dp/B000J3RSW6
You can buy blue chalk or white.
It is supposed to be easy to use. I haven't tried it yet, but I know my LQS sells it for $20 for the set, so this is a good price.