I actually found this one out, quite by accident. Long lonnng, ago, I was working on a doll. At that time, I was staining my doll parts prior to stuffing. I grew impatient, which is just one of my traits!, and while my doll parts were still damp, I decided I could wait no more. So I began to stuff. What I found was that the stuffing was more even, the seams filled out nicely with no "ripples" or "puckers" and when it all finally dried, the fabric had shrunk, ever so slightly, getting rid of any tiny wrinkles that still existed. I now had a firmly stuffed doll with nice rounded head, hands and feet.
I adapted this to the way I now work, After the doll parts are sewn, prior to turning them right side out, I lightly spritz them with a water spay bottle (Walmart has them cheap for about .99 cents in the section where get all those little travel sized toiletries).
After you turn your parts, spritz again. Stuff the doll right away before it begins to dry.
Oh, one more thing, I ALWAYS wash and dry my muslin prior to cutting and sewing. It removes and kind of "finish" that is on the fabric.
Hope this helps someone!!