as part of my initial learning curve, i was basically not using my gears for the first month or two i was riding. not my brightest move! but i was concentrating on other things like traffic, endurance, etc. i caught up eventually!
now i realize that many many factors go into my gearing, including my initial speed, my energy level, the size of the hill, traffic, etc. it's very complex, and i'm sure it will be a couple years before i always know what i'm doing. sometimes i still look down at my gear levers and wanna smack myself, 'yeah, that's why i'm a little tired. duh!' don't worry, it happens to all of us!