like the others said, it's all about networking. don't bother taking out large adverts in the paper as it's of little use.
treat your existing clients well. you'll find a lot of new business will come from them.
a guy at work did what you're doing and went freelance first, which was a bad idea and he didn't have the customer base to survive. you'd be better off working at an agency first in a junior role. once you build up a rapport with the customers there you'll find freelance business will come in quite nicely.
best of luck
