+1. For hills that push me to the limit, I stay in the seat and pedal at a higher cadence. If I have anything left in the tank near the top, I'll go up 2-5 gears (depending on how much I have left in the tank, and how many more hills I have to get over that day), stand, and mash to the top. I like the feeling of powering over the crest rather than limping over it.

For easy to medium hills, roughly half way up, I go up 2-3 gears, stand up, and put out a burst for 5-15 seconds before dropping the gears back down, sitting down, and spinning again. This is especially effective for me during a short, steep stretch. I'll then stand again near the top.

As others have mentioned, this is largely a matter of personal preference. I used to HATE standing, until I got comfortable with the right gear choice. If you stand up without enough tension in the pedals, it feels all wrong. And if you stand up with too much tension, your legs fall off.