It so depends on the hill...but for a very steep climb I find that an upright position with my hands actually close to the stem (on either side) seems to be helpful. On longer slower climbs I've had more luck being a bit less upright, or even in the drops (which allows me to focus on charging up the hill vs. how long, boring and uphill it is going up the hill).

I do have a tip for descents....

Even if you're sucking in air from the hill you just busted up...keep your darned mouth closed! (she says from experience)