From Jin Saotome:
What you need: Super Glue and water.
What you do: Apply a drop of super glue into the joint. Work it around so it's all inside there and so it doesn't instantly set up. Then pour, dip, or drip water into the joint. Move the joint while you're doing this. The super glue instantly crystallizes into a matte-clear substance and expands, filling the joint. Let it set for about 50 seconds and then test it. A couple crumbles of dried glue will come off and the rest will be tightly adhered to the sides of the joints. Blow into the joint to get any water out afterwards.
Yep, it's that simple! I tried this on Jin's knees and ankles, and on my Colossus's torso. I wouldn't risk it on my favorite figure if it didn't work. Right now I'm trying different glues. Duro Superglue seems to be the crustiest when water is applied. Future Glue seems to crystallizes the best. But the regular Super Glue brand stuff (not Duro) works pretty well depending on how clean the inside of your joints are. The more mold release in there, the less it holds. Experiment with the super glue and water with something else if you want. Smear the glue on a junk plastic part and dip it, or put a drop of glue in water.