We have tried several different ways to make dummies. The major problem we have is that whatever we do need to be somewhat portable. (although we would love to own a few mannequins). Since the haunt is getting bigger we decided we're going to try PVC frames this year.
Some of our designs worked better than others. We put a mask on the top of a ladder , pull a black robe over the top and wrap a black cloth around the bottom. It makes for a really nice 'giant'. His only shortcoming is that he has no arms.
We've tried stuffing clothing and attaching the stuffed dummy to poles (to hold them upright). For heads we pull masks over styrofoam wig heads. The shortcoming here is that you end up with a limp pose that is decidedly a dummy.
Now, one of the things that worked best, actually started out as a practical joke. One year for Christmas, a few of the guys at John's office gave him an inflateable 'whoopee' doll. The only problem here is the same as the one below.
We had one of those costumes that looks like someone is carrying you on their back. Part of the illusion involves an inflateable torso that comes with the costume. We usually put this on a chair (it's our witch staring into the cauldron). The shortcoming of this one and the previous is that as inflated things get colder (as they will in NY in October) they shrink. It's physics, you can't do much about it. Also, with age, they tend to sprout tiny little holes (usually around a seam) you can't seem to find.
When we build our PVC dummies, we'll put up some pictures if we decide they work well.