I haven't ridden without a computer in years. Back when I started I used to trace the routes I rode on the Denver Bicycle Touring Club's route map using a straight-edge and then tally up the miles based on the map's legend. That got old pretty quickly, but until I got my first bike computer, I never failed to keep track.

All of mine are Cateye. I've tried others, but Cateye seems to be the most reliable...