The frixion is my favorite - as others have said - use a hot iron on them. People have said that on dark fabrics the lines turn white. They also say that washing it will cause the white line to go away - I have never gotten around to testing that by washing it. Someday....I like it best because if I start marking and change my mind or realize that my vision isn't going to work - a quick swipe w/ a hot iron allows me to start over.
People complain about the frixion coming back when quilts are put in freezer - but I won't have my quilts in really cold places. But when i wash the quilt when I'm finished - the pen comes out anyway...
My 2nd favorite is the Fons and Porter pencil. I like the white lead to mark on the black fabrics...
As w/ anything used - always test it first.