When in a psych class in college, I learned that the hands in the pockets is a nervous/defensive mechanism. Hides the contents of the hands, as well as offeres a sort of protection to the softer parts of the anatomy. Crossing the arms about the chest was also noted as a defensive measure for similar reasons.

I've found it to be more effective and comfortable to hold your hands behind your back, hooked into your belt if standing for a long period of time. This pose, if I recall correctly, is one of overt confidence, and ( in some circumstances) defiance...as if you are offering your more sensative parts up, daring one to advance.