Getting Started is definitely the hard part. I was just talking about this with my girlfriend the other day and she brought up a good point. I was talking about how frustrating it can be when people ask me about commuting everywhere by bike and I tell them anyone can do it. Their response is a resounding "wouldn't work for me". I think it is a shift in thinking.
Why did it work for me? My girlfriend and I had moved to Central America for about a year before returning to the US. While in Central America we became very accustom to getting around via foot, bike, or bus. When we returned to the US, even though we share a car, the first thing we think of when walking out the door ISN'T the car. Our car sits idle in the street. But this shift in thinking came over a year away from the hub bub of the automobile in the U.S.
So I think that besides the real troopers here who have gotten to car-free without the separation, it is difficult to get to the mindset of going somewhere without your car.