How can I loosen my Minifigures arms and legs?
Both arms and legs are held in place through a fairly simple compression pin.
A small dab of washing up liquid applied to the joint should lubricate it quite well - avoid oils as they can cause discolouration of the plastic.
If that doesn't work for you, then you could try physically loosening the pins - gently pull the arms out of the sockets and the legs off the hips and replace them a couple of times - this is usually enough to compress the arms of the pin enough while still maintaining a grip on the part. Going further you can obviously compress the pins manually and fairly permanently - however this will likely make the joints too loose for your needs - I've certainly got some older minifigs with very wobbly hip joints that can no longer stand up on their own.