Even without a strict training regimen, watching power numbers will tell you if you're getting stronger, and coupled with a heart rate monitor, you'll know if you're getting out there and doing the work to improve, or just putzing around.

Knowing that alone will act as a motivator, and it's quite simple to fix goals to drive progress, for example, to just hold a few watts more up a familiar climb, or push a little harder on your favorite flat.

Do those kinds of things regularly, and it adds up to something, especially for folks with an essentially lazy personality type like I have! I seriously can't tell, without metrics, whether I'm working hard or feeling lousy, so seeing the numbers during the ride really helps me go harder.