Stiffness is a pretty basic concept and I don't think it would require an example.
Stiffness is avoided by having LOTS of movements, everything moving all at once, this makes your figures appear "loose" as they do in real life.
You can establish this by moving EVERY node in EVERY frame even if it's just by a little pixel. This rule should have very few exceptions. There are very few cases in which a node can or should remain static. You should be able to pinpoint these exceptions easily enough.
Be careful when doing this though. Moving nodes around in random directions will just end up making things look shaky. Try and make sure that all of your movements have good direction, and make sense.
Actually, my avatar is a good example of stiffness. The arms, body, and head have zero movement. I've been thinking that a very slight shakiness and breathing motion would fix this. But that would be hard
Anyway, always remember to ask yourself, if someone were to be doing this in real life, is this generally how it would look? If the answer is no, you know you're doing something wrong.
To sum it up quickly, move everything. Might look a little weird at first, but you'll get the hang of it.
Hope this helps and have fun animating.