The first thing I do when I walk in the door from a bicycle ride — even before a shower — is to record my time and miles in a bicycle log.
I like to keep it simple. Route, miles, time, MPH, and comments. I've created an Excel spreadsheet file that computes the MPH and tallies the monthly miles and hours in the saddle.
Most of the other bicycle training logs I've seen (here's another free one at Arnie Baker Cycling) include reams of data points for such things as stretching, resting pulse rate, elevation gain, cadence — you name it. Many of today's bicycle computers can record this data, some can even download the information right into the computer.
That's great if you're training for the racing season or if you're serious about meeting other bicycle training goals. But for me, I'm just curious about how many miles I've knocked out for the month.