[evgen]

Try a whirley-pop popper. It has vents at the top to let out the steam and a hand-cranked rotor to agitate the popcorn as it is popping so that the unpopped kernels fall to the bottom to finish cooking.

[jcoffland]

I find that I can get near 100% yield with only minimal agitation in a regular pan. The popcorn popping provides plenty of agitation on it's own.