Try the
Kool Stop Tire Bead Jack (you may have to scroll down a ways to find it). There are still some tires that I struggle with with this tool, but they would be impossible without it. Park also makes some steel tire levers that might give a little more leverage with stubborn tires.
I've noticed that thick rim tape (like Velox cotton) can make it harder to get a tire on, although the extra durable rim tape is nice to have once the tire and tube are in place.