I've been a freelancer off and on for the past few years. I'm 22, in university, and have a start-up habit, so I need a way to fund myself. I found a great niche for myself in the past month that allows me to focus on delivering great work at breakneck speeds.
My main focuses are to be fast ("wicked fast", "what do you mean you're done already?", etc. from some clients since), to deliver great quality, and to deliver more than I promise, and act beyond professional--yet not quite too friendly--to clients.
The result is amazing. I'm already at a stage where I need to decline work and were I to be doing this full time I easily bring in enough requests to amount to what I've totaled as $525~ worth of work a day, which I could complete were it a full-time gig.
It's very easy: Make a dead simple portfolio that lets your work speak volumes where most people use gimmicky words (mine's at http://sicret.net ). Make your clients proud of your work (a lot of my clients love showing off their new designs, even though it's an industry that typically keeps things very private for competition's sake). I'm brutally honest with a client as well. If I am running behind because of some other mix up or I am not satisfied with a design, I will tell them, and offer them a discount or to redo.
Customer satisfaction is more important than customer acquisition. So is being truthful.
Don't be afraid to drop ineffective clients. You're in it for business, not to make everyone happy. your happiness comes first.
The most important thing I've learned: constantly charge more. Eventually you'll plateau, but until then or until your own project takes off, keep learning and keep getting better; then keep charging more. You'll be worth every penny if you give half a damn.