This fix will work for both 12 inch scale as well as 3 3/4 inch scale figures. Please note, if you plan on selling a figure with this restoration, it should be noted in the description. The common method goes as follows:
Drip or brush a small amount of clear ELMERS GELL GLUE into the loose joint. While the glue is drying, move the limb back and forth, until all the glue has set. This way, the limb will stay mobile, yet the dried glue in the joint will give the joint more resistance, and make it not as loose. I have done this on a loose 12 inch Boba Fett with great results!
The Beauty of ELMERS is that it is water soluble, so a good long soak will loosen the joint, should you over glue it or wish to undo the fix!