Try to learn to spin at 90 to 110 rpm, you only need to check it once in a while because you'll be able to feel when it's right. Watch Lance in the tour, he rarely goes below 100.
When you're trying to ride hard (after you learn how to spin) you'll be able to go faster on a lower gear spinnig at 100 to 110 rpm than when you're mashing at 80 or 90 in a higher gear.
I can hold thirty in my 15 for a mile or more, but have trouble doing 28 at all in my 13. I only use a 12 or 13 when I'm sprinting very fast, (that's pretty much history now) or decending.
If you spin fast and don't jam or push your gears you'll have far less injuries too.
One of the first things I learned my coach when I was racing was- Speed comes form being able to spin your pedals, not from mashing a big gear.