I also use Pigma pens and then heat set. One caution, make sure that the fabric they are signing has NO fabric softener in it or spray starch on it. These will cause the ink to not be permanent. (The fabric should be prewashed, but don't use fabric softener.)

I have had great success with the pigma pens as I use them to sign all my quilts. Even my first quilts, after many years and many, many washes later, the ink is holding up beautifully.