You need to get in a situation where you get to know customers and other people in your area, BEFORE you leave your job.
When I left my job (I ran the shop), I gave a years notice, and told the owner I wouldn't even talk to any of our customers for a year after that. My partner was already getting things rolling while I was still working in the sweatshop, so we had jobs lined up.
Long story short, I made it 5 months into my year and simply couldn't take the crap anymore. It didn't end nicely. Within 2 weeks, the customers from the sweatshop where knocking on our door. I still won't talk to them, but my partner does, we never undercut a single job, and went up in many cases, and still got it all, this is WITHOUT solicitation, based solely on reputation. Two customers even went so far as to cancel POs and pick up material from the sweatshop.
It wasn't my intention to take work away from the sweatshop, but when they can't buy tools or fix machines and cut peoples pay, not a lot gets done. Now the Sweatshop owner is trying to get us to do work for him, since they can't "handle" it. Mostly a lot of the nasty tricky I had done when working for him.
If I had to start cold, not knowing anybody, or having anything lined up, I wouldn't have done it. Everything just came together at the right time, from my partners divorce, to some damn near free machines to the Sweatshop slowly sinking like the Titanic. It was like one of those "perfect" storms, but in a good way.