Great question. There are a couple of different things you can use. But, test them first.
The thing I like best, because it usually rubs off before you finish quilting the quilt, is a chalk pencil. Roxanne International has some great chalk pencils and come in white and silver.
http://thatperfectstitch.com/products.html

Soapstone is great, although it can't be seen on a white quilt. It is available in quilt shops and at amazon.com
An alternative is Berol Verithin pencils - in silver or black. They, too, rub out.
I would stay away from the blue markers that are sold in quilt shops. I used that once on a quilt, and it never washed out.
Hope that helps!
Penny Halgren
www.How-to-Quilt.com
