When I did some work at Goddard, I knew this guy who would remove heatsinks by throwing the board into a zero-moisture freezer, then drop the temp down till it was almost dangerous. At the same time he'd be heating up a big frying pan on the break-room range. Once the pan was really hot, and the board(s) was really cold, he'd take the board out of the freezer, turn it over with the heatsink down, and press it into the bottom of the frying pan (no oil). The heatsink would soak up the heat, and with the rapid change in temperature extremes, the sucker would just fall right off.
Real sweet stuff.