Many people swear by the Frixion pens. However, the company states clearly on the package that they are NOT to be used on fabric, so I question what future damage these pens will cause to the fabric. (I think about this when I see antique signature quilts that have holes where the signatures used to be - the ink has corroded the fabric. Will this type of damage be the same when using the Frixion pens? They haven't been around long enough to tell.)
Soapstone pencils work well. Chalk on dark fabrics works, too. I use pencils especially made for quilting (white, yellow and gray). I bought them at my local Walmart for under $5 for the three.
Hope this helps.