Strange pucker in the Multiresolution modifier

I’m pretty new to blender and I have been playing with the modifiers to get a handle on how they work and often times I get puckering and creases in odd places when I use the Multiresolution modifier or the Subdivision surfaces modifier. In edit mode and before applying the modifier I don’t see anything that could be causing the problem. What is going on?

In this particular model I got a pucker in the character’s front and back and then a nice crease that ran from one pucker to the other. If i pull the vertices in the offending area out it decreases the effect but also give the character a hunched back or a huge gut and it doesn’t even fix it all the way anyways.
Any help would be appreciated.

do you use mirroring? It could be that the origin is not positionned on the edge that is the connecting edge for the mirror. Also do a Ctrl A to apply rotation and scale. Another possible issue is that you have duplicated vertices or edges. Do a W remove doubles vertices.

Remove double vertices (select all and W / remove doubles)
Remove any faces or edges that are inside the mesh.

Suppply a link to your blend file for review

Brilliant! It worked. I removed the double vertices (it turns out there were 8 of them), but that still didn’t do anything. Then I looked inside his head and sure enough there was a line of faces that followed along the crease. After I deleted all of them everything turned out nice and smooth.
Yes I used mirroring. How do I make it so that it doesn’t do that when I mirror? And how on earth did those faces even get in there? And for future reference where would I put a blend file such that I could have a link to it so it could be reviewed?