For short, steep hills, I'll usually shift to a gear like 48x20, and get out of the saddle, and try to maintain about 15mph. But for me, this only works for things like freeway overpasses, etc...6% grade for 1/8 mile or so. Longer hills, I'll drop to the lower chain ring (34t) and find the cog that works best for the hill I'm climbing, in order to keep my cadence up, and not lose too much momentum.