All my really big hills are proceeded by smaller inclines so its difficult to build up any speed. However, once on the hill and in my low gear I pedal hard with one leg which allows my other leg to somewhat rest, and then pedal hard with the opposite leg and repeat.
Its basically push hard with right leg (which allows my left leg to just spin) and let the crank spin and then its push hard with left leg (allowing my right leg to spin), and repeat.