SO there's questions of measurement, accounting, and motivation - three different things, three different challenges.
Measurement. I love love love exactly the same computer Salty refers to, Cateye Astrale8. I have one on each of my three bikes - even though you can set one computer up to cover two bikes, and buy an extra wiring harness for $19.

This is a great computer, I'm a cadence rider (I'm 85 rpm plus-minus 3, I really don't care about my speed) so it's got everything I want and nothing I don't want. The GPS units tend to generate virtual cadence which is a secondary derived number and that has issues.
Accounting The second issue you raised is tracking miles. I suppose a simple technique would be to just zero-out the accrued mileage on the computer every Jan-1st (hey, how convenient, right around the corner!).
Motivation An athlete friend said they tell people planning their first marathon: tell all your friends you're going to run a marathon, don't keep it a secret. Nobody wants to be a dog in front of their friends. It works.
Personally, I need to ride my bike or they're going to cut my toes off (diabetes). I like my toes, they help me count between 11 and 20.
Writing about my miles and my rides has been an effective technique in keeping me on the bike. Every day I ride, I write about the mileage the weather, what I encounter, I show a picture, and I document the mileage and my weight. Every Saturday I present an end-of-week summary including trend data.
So maybe you might Facebook or Twitter your rides and progress, with a picture or an anecdote from each ride? You've already got a following of people who know you, just leverage it.