|When pin fitting a muslin, use safety pins. As you achieve a fit that you are pleased with, close the pins in that area. Your alterations will stay secure and you have a sure mark to tell which areas are fit. Areas which need further 'tweaking" remain open and ready to be easily changed.
I feel much better about taking a muslin on and off many times when I know that the pins are secure and I'm not in danger of becoming a living pincushion!