Like everyone else, I often stack similar sized plastic buckets,
but, over time, they almost always become close to impossible
to pry apart without excessive force and undue swearing.

http://www5.picturepush.com/photo/a/...g/12718448.jpg

There must be a simple "trick" that prevents them from sticking
so tightly that one has to bang and curse to pry them apart.

What "trick" do you use to keep your plastic buckets from
sticking together?