Steve Pavlina has written a guide detailing 10 common mistakes made by the newly self-employed, like spending too little money.
Don't let frugality get in the way of efficiency. Take advantage of skilled contractors who can do certain tasks more efficiently than you can. Buy decent equipment when it's clear you'll get your money's worth. You don't have to overspend on fancy furniture, but get functional furniture that helps you be more productive. Don't use an antiquated computer with outdated software that slows you down if you can afford something better.
If you've given yourself the freelancer self-assessment and you've decided to give it a go on your own, you should definitely check out Pavlina's 10 mistakes.
10 Stupid Mistakes Made by the Newly Self-Employed [StevePavlina.com] [Lifehacker]