A laundry pen might be good. My grocery does not have them but they may be available at a sporting goods store. Also Sharpie now has a "permanent" pen but I have not tried that yet. Check the label. Sharpie and Pigma pens both seem to fade on my sample pieces that are washed several times.

I press butcher/freezer paper to the back of label to give it stability, then write on it w/ Sharpie Fabric Markers that I bought at Hobby Lobby. This does not fade w/ washing. I used them to make an autograph type quilt for the building principal last year.