OK you have to be REALLY careful, but basically you drop one or two drops of superglue into the joints and then work the joint until the glue dries. The superglue then sets in the cracks of the joint and adds resistance to the joint. It can take a while for the glue to set when you're moving the joint around but it does eventually work.
I'd suggest testing it out on another figure for your first time. If you leave the glue set it can take some work to "break" the glue and get the joint to move again.
I now have a really poseable He-Man when before his leg ball joints kept making him do the limbo.