I quit smoking in 2003 about a day or two after passing out on the interstate feeder road while taking the family to Chuck E. Cheese's. While the passing out certainly played a role in the quitting, it wasn't "the" reason I quit. It was time.
I firmly believe that when it comes to quitting something, you need to be physically and mentally ready to quit, both on a conscious and subconscious level. I think there are several people who "want" to quit but are not ready and therefore their attempts to quit will not be fruitful. Once you're ready to quit, though, then it is rather easy - you essentially just stop.
I had a similar situation happen to me yesterday - I made the switch from regular Coca-cola, which I've had virtually every day since 1995 (and a whole lot before that, as well, but '95 was the last time I stopped drinking it), to diet Coke. While I don't expect merely making this switch is going to result in my losing copious amounts of weight, I do think it will help stem the tide of weight gain I've been noticing over the last couple of months.
I still need to exercise more (I looked at Bikram yoga, but I'm a little too Middle Class Midwestern at heart to make that plunge, especially at $140/month), and might start running in the mornings, or something to get on the way down...
But it's a start.