One sneaky trick is to lay the wheel/tire assembly down flat and apply a liberal dose of Diesel fuel to the tire/wheel junction. Let it sit a week, flip it over, and repeat on the back side. The fuel softens the old rubber, and loosens the rubber/steel bond at the bead seat area.
If you're going to be doing more than an occasional tire change, investing in a slide hammer bead breaker is a good idea. It looks like a 4 foot long piece of pipe with a duckbill bead breaker on one end and a sliding solid steel bar inside the pipe. It's much more controllable than a tire hammer, and it doesn't beat up rims. It also reduces the possibility of back injuries from swinging that big hammer!
http://www.gemplers.com/img/ken-toolreg-heavy-6H-lrg.jpgJerry