It's probably completely wrong to do so, but I find I can get up almost any hill by varying my cadence. I might start out spinning an easy gear, then just stop pedaling and coast until I almost come to a stop, then get up out of the saddle and mash until I'm back up to spinning speed to sit down again. It's probably inefficient as all hell but it works for me. I also upshift when the grade eases to build some speed.