Usually 39T in the front, and somewhere from 13T to 25T in the back, depending on the terrain. When I'm going down hill or making time on the flats, 53T in the front and fewer than 20T in the back.

I change gears when I need to ... I guess it's more often than some people I ride with. Not all the time, mind you. Basically, if I'm spinning the pedals too quickly, and my heart rate is too high, I'll get into a higher gear; if I'm pushing too hard, and too slowly, I'll get into a lower gear. Pretty soon I'm in a happy equilibrium.