One day, it occurred to me to test the washable markers sold for children's art. I bought 3 packs at National Bookstore, draw some lines on a piece of white fabric I had lying around, ironed it (heat sets some dyes making them more difficult if not impossible to wash out) and threw it in the washing machine. I was really happy to find that all the marks washed out!
Now, not only do I save quite a bit of money as I don't have to buy expensive quilter's pens from abroad, but I also have a selection of different colors. This can be really useful if you are having many fitting challenges, and aren't sure which was the latest change. I use a different color every time I have to mark a new sewing line so always know which was the latest adjustment!
WARNING: Even though I have been using washable markers on all sorts of fabrics for some time now, I still test every new pack I buy and would highly recommend you do the same before drawing all over your beautiful fabric with them. You never know if a manufacturer may change their ink recipe, or if an ink reacts to the heat from an iron differently on some fabrics. I also keep my pens separate from my daughter's art supplies and mark them with some colored tape, so I don't accidentally use one of her permanent markers!