If it's a short climb and I think I can get up it without losing speed if I stand, then I do. If it's a long climb - I sit and spin to the best of my ability. Sometimes, I mix it up - spin where I can, stand if it gets too steep, go back to spinning, etc.

Basically, the first year I started riding, I stood all the time. Then I found that my muscles got used to it and I couldn't effectively spin. My second year riding, I tried to stay seated more and just spun. Then I found that by the end of the season, standing would wear me out because my muscles had gotten used to spinning. Now I try to mix it up - partly to keep my body capable of doing both with some efficiency, and partly because I've gained a bunch of weight and I need all the tricks of the trade to get up hills!