I use 45 seconds in a 1250 watt microwave, then, being careful not to burn my hand on the hot nut, crack carefully along the to seam using adjustable vice-grip pliers adjusted so that they don't crush the kernel when the shell breaks and the pliers slam shut.
3 or 4 cracks that way usually opens it up enough to extract the kernel intact. The microwaving seems to release the kernel from being stuck to the shell so that it comes out fairly readily. The vice-grips give you plenty of leverage if you use a large one and its adjusting feature lets you exert as much force as needed without concern that when the shell breaks, you would overshoot and crush the kernel.
A rubber band wrapped several times around the end of the handle and holding down release lever prevents the pliers from locking closed, allowing you to sort of "nibble" your way along the nut's seam without having to let go and release the pliers before taking the next "bite". The workshop vice, suggested above, would have the same advantage, but most people don't have one mounted on their kitchen table.