I love the Clover fine line white pens. They cost about $7. They leave a clear white mark, and the point is fine enough to fit in the track of a stencil. It takes a few seconds for the line to develop, which is kind of weird. It doesn't rub off while you're quilting. You can remove it by ironing over it or by washing the quilt.
I used this to mark a project I'm hand quilting. I marked it in December, and the marks are still clear. Where I've quilted, the marks are not as clear, which is a good thing.
Janet