Seriously. I'd like to hear more about how you managed to quit.
I think the impetus I needed was the right business. I'd been working on Virtualmin for fun for two years...somehow it never occurred to me to make it a business for most of that time. When it finally did, it all seemed inevitable: it really felt like, from day one, that as long as I worked hard, it could not possibly fail (and that initial gut feeling is turning out to be right a couple of years later).
Some things I did to make it clear to myself that I was done with that old business:
1. Gave up my downtown Austin office. (Corner of 6th and Congress, Bank of America building...best location in the whole city.)
2. Took down the old business website. I left up all of the support materials and such (and it still gets thousands of hits per day), but I removed all contact information.
3. Gave myself deadlines for launching the new business. "The website will be launched on X, and we'll be selling products by Y."
4. Did Y Combinator. Not everybody gets to do that, but it triggered a number of changes that made the break between the old life and the new one very dramatic and difficult to go back on: I sold my 350Z and had a huge garage sale to get rid of a lot of my stuff, moved to the Valley (the getting rid of stuff is enforced when moving to the Valley--I couldn't afford as much space out here as in Austin), and didn't keep my old phone numbers.
The last one has probably been the most effective. I was still occasionally taking jobs when friends asked when I still lived in Austin. The thought process is, "Hey, what can a couple of days hurt? It'll give the new company more runway." But it slows you down, and gives you a safety net. Once I didn't have the safety net, I started thinking a lot faster and lot harder about how to increase revenues to a livable amount (I've never had a lot of big expenses, even the Z, so I had enough savings to last quite a while, but moving to the Valley dips into that pretty quickly).
NOTE: I don't have a family to feed and house. My co-founder kept his full-time job throughout all of this, because he has a family. I'm not saying the dramatic "drop everything and get serious" is the right path for everyone.