Just to add a little to ideas of earlier commenters regarding using an overlay, there is a product called Duralar which is a clear plastic sheeting that some quilting shops are selling in cut pieces of about 20"x24" that you can use for trying out different ideas by laying it over the quilt and drawing on it. Duralar can be purchased in big rolls if you want that much. Until a recent course I attended, I'd use a piece of plastic, like wrapping paper plastic or a rigid plastic insert from packaging and a dry erase marker as already mentioned. But in the course I recently did, she used a water soluble pen, like you would use to write on a transperancy and it can be used on the duralar and also on rigid plastic (a piece I bought at Home Depot) and until you wet it, it doesn't come off. I like that, as the dry erase always produces a dry marker dust that is easily transferred to your hands and the quilt. Anyway, I also use these overlays to test out ideas for quilting as it gives you a really good trial run on figuring out your quilt path and scale.