I'm assuming your fabric is 100% cotton. I have found the towel trick really useful. If I'm writing on fabric (like signature blocks) I iron freezer paper to the "wrong" side. It provides a really stable area to write on. Freezer paper can be found at any grocery store. Make sure you always have the iron touch the dull side (the shiny side is waxed and will make a mess on your iron, guess how I know).