I think that breathing is underrated. I find that making sure that I am breathing with my belly muscles as well as the diaphram really helps. Check Bicycling Bliss by Portia Masterson for more details.

Other than that, use as low gears as are comfortable to spin, and practice a lot. My commute is all hills. On the big hill on my commute I used to struggle up pedaling slowly in low and now I am spinning in one gear up.