I stand while climbing on the steeper hills. But instead of using my weight, I sort of get into a crouched position--moving my body forward, and then try to pedal as aggressively as possible to get to the top as quickly as I can. I find that standing up while pedaling (anytime) helps keep the blood flowing around the sitbones, so it results in a more comfortable ride. Climbing out of the saddle also exercises different leg muscles, which I also find to be a nice change to break up the ride.