don't use Dawn dish washing liquid in the formulae above. I am told it has a chemical in it that will set the marks.
I have used a Pink Pearl rubber eraser with some success on pencil marks. It is soft and clean. I find sometime the white "fabric" erasers pick up some of the graphite and smudge the rest of the area.
The formulae works I am told. Do as suggested above and then blot with rag dipped in clear water and continue until all out. Many hand quilters swear by this.
There is also a product made by Quilters Rule for removing marks.