How do I make a stiff comforter softer?

The next time you take it to the laundromat, add at least 1-11/2 cups of white vinegar to your rinse water. It will cause the fibers to relax and soften while it will lock in the color and keep it from fading over time. I would still put a couple of clean tennis in with it in the dryer and use fabric softener sheets in the dryer as well. Don't worry the vinegar smell dissapates in the dryer.