Originally Posted by Helmet Head
What you want to develop is the ability to ride with a high cadence, in the 80-100 rpm range, for normal flat riding. It feels strange and uncomfortable at first, but once you get accustomed to it, you'll find you're getting from A to B faster with less effort.
My own experience backs this up 100%. I started bike commuting this past July, after not having ridden a bike for over ten years. The first few weeks I ground the high gears and got a serious workout, but not much speed out of it. Then I discovered the magic of high cadence. By clicking down one or two gears from where I had been riding, each stroke was easier but I could go faster with what felt overall like less effort. The secret I think is in the cardio--once your body gets accustomed to regularly exercising at a heightened heart rate, you can hold it there a lot longer.
Like some others who have posted, I tweak the gears to keep my cadence and effort where I want them. Sometimes I want to really crank it out and will go to a higher gear, sometimes I want to take a little breather, or hit a hill or wind, and go to a lower gear.