The method you use very much depends on the type of figure you are working on, for 4″ Action Force/GI Joe type figures you can simply replace the “O” Ring, for others I find a small drop of Pledge Floor Care Multi Surface Cleaner on the inside of the joint does the trick nicely, but you have to leave it a day to set and you also have to make sure that no liquid spills are left on the plastic as it will melt it.
For removeable limbs you can pack the joint with a small piece of tissue paper, re-attach the limb and it will be nice and tight.
You can also use PTFE tape which is a stretchable tape that plumbers use to seal the threads on joints.
Another method is to use PVA. Coat the joint then move the joint around to spread the PVA, then leave it to dry.
