It sounds like you're staying in a squat rather than standing completely up. When you stand to climb, you'll be extending your knees almost fully, really standing up except for whatever bend in your hips you need to reach the handlebars. Commit yourself to the motion, it's really just like getting up out of a chair initially, and then, as others have said, like riding an elliptical machine once you're standing.

The conventional wisdom is that standing takes more cardiovascular energy. But sitting and spinning takes more leg muscle, and sometimes we run out of leg muscle before we run out of cardio capacity. It's good to be able to climb a hill either way, and to be able to switch back and forth on long climbs.