I use Excel currently. I rely on my calendar and go through my appointments and log mileage. I'm missing a lot of stuff though, because it doesn't land on my calendar. Post office stops, bank stops, etc. Things that are easy to forget. I don't drive a lot, so I estimate that probably 75% of my mileage is business related - however, my mileage log indicates that I'm actively recording a lot less than that.
This is why I need an app.