I'm a bit late to the party, but what keeps me motivated is setting a consecutive commute goal.

First, it was to see if I could go a whole month without driving to work. Once I hit that goal, then I set a goal for 50 days. After that it was 100, but on day 98 we had a monster snow storm that stopped me, so I had to start over.

The next time around I successfully hit 100 days, so I went for 150. Once again, a snow storm intervened on day 146.

Determined to hit 150, I started over. After I broke that personal record I just kept upping my goal by 50 days. I'm currently on day 316 and counting.

And by now it's so much a habit that I really don't seriously think about the possibility of driving to work.