I just discovered Mile IQ, and I'm loving it!
It automatically tracks your mileage on your phone. You don't need to remember to open the app up before heading out - just keep it running in the background on your phone and it records your trips.
You can go into the app at any time and classify your trips as business or personal, and each trip shows on a map so it's easy to remember what the trip was.
If you're self-employed, you probably realize the benefits of tracking mileage to claim as an expense on your taxes. Now there's finally a simple way to do it.