Okay, maybe not everything, but you get the idea... Here's a screenshot of a jumpsuit I'm working on for V4. It has JCMs, but it hasn't been magnetized and the joints are not perfect yet:

The top of Vicky's thigh pokes through, and the joint is screwed up because the joint parameters aren't perfect, but the underside of her rear (circled in red) is smooth. Now I apply the magnetize to V4 pose, and...

Now the top of the thigh doesn't poke through, but there's a bunch of jagged edges under her rear. Does anyone know what causes this, and how I can get it to go away?

How is it grouped there?  Did you use some auto grouping utility to group it?  From the colors I'd guess you have it grouped the way the colors are arranged, which is going to give you problems (left leg is going to bend the right buttock and hip e.g.)

Because the figure is modeled and rigged in a T-pose, that area is a pain to rig well for most types of garments, and in any case you pretty much have to group the polys as closely to how the figure is grouped as possible for best results.  Note "best" here means "rigged as well as the base figure", there are better ways something could be rigged, but your rig has to follow the base.

The picture shows the hip and thight groups for V4, clothing should approximate that grouping.

Wow, sorry it's taken me this long to get back here! I had forgotten all about this post.

The colors do not represent the groups; the actual hip/thigh grouping looks like the picture markschum posted. However, I think you guys have put me onto what is wrong... I grouped the jumpsuit with OBJ2CR2, and the backside of the group looks like the front. Now that I look at Vicky's grouping, the thighs come up higher in the back than they do in the front. I'm guessing that there is a magnet in either the hip or the thigh that is set to only affect that body part, however, it's affecting polygons it shouldn't because the group contains polygons it shouldn't.

I'll have to go in and regroup it. sigh I have no idea how people rig anything from scratch. Vicky and her joints can go fly a kite right now as far as I'm concerned.

Don't bother trying to use Poser's group editor for this purpose, it's an immense waste of time and effort that way.  What modeler do you use?  Do your grouping in the modeler, typically you'll have access to many modes of selecting polygons that make this pretty simple.

I model in Wings 3D, and I've experimented with grouping there. It's a little tricky, though, because as far as I can tell it doesn't directly support polygon groups. I've had to use different materials to establish the groups, so it throws a kink into my plans when I want materials zones that are different from the body part groups. I'm sure there's a way, I just don't know it.

I don't have any advice for you, I've never worked with Wings 3D.  Maybe ask in the wings 3d forum.