As above, proper technique helps a lot. Make sure your tire beads are in the bottom of the rim well (repeat as necessary). Wrestling the last 6" onto the rim is best done with the heels of your hands, and if that doesn't work, repeat the beads in the bottom of the well step.

My all-time favorite tire irons are the Soma steel core irons, though I only use those for getting the tight tires off the rim.