A few key rules of thumb that have worked very well for me are:
1) Keep you debt as low as you can, and pay for things in cash whenever possible. You can be far more flexible in terms of location and type of job/salary if you aren't worrying about a mountain of debt. When you make money, invest it back in your business -- supplies, training, equipment, whatever you need to keep you up to speed, but don't overdo it. Pay off the debt you have as quickly as possible, even working a second job to do it.
2) Do a regular cost analysis and have a good understanding of how much it takes you to operate your business -- rent, utilities, taxes, etc., down to the paper and ink you use in the printer. Add on a decent profit plus another percentage to cover surprises, and stick to your rates. If you don't, the only person to lose out is you. A good book to help you develop good business practices is: http://www.amazon.com/Graphic-Designers-Pricing-Estimating-Budgeting/dp/1581150989 and there are others -- the important thing is to think of yourself as a small business, and learn how to run that business profitably.
3) Learn how you work, and make that work for you. By that I mean figure out if you're a night owl, a morning person, do you need a pot of coffee on hand, do you need to take a break every hour, do you like books-on-CD, do you like silence. Whatever it is, make that the way you do your work. There's no need to fight against your own nature, you won't produce your best work. Figure out what it takes to get you to sit at your drawing board or computer for the hours it will take to complete the job, and do that!
4) Don't do spec work. Ever. Your time and talent has value, and people often only value you as much as they are paying you -- sad but true. Artists are about the last people in the world who can afford to work for free, but that doesn't stop the sob stories. I tell people that I have the same rates for everyone, because I want to do equally good work for everyone. If they can't respect that or afford you, remember that you are the only one who loses if you take the job at a low rate or do it for free.
5) Put your title (illustrator, graphic designer, etc.) and contact info in your email signature, starting now. Use it when you write to everyone. It will remind people who you are and what you are up to -- you never know what contacts your families and friends may have, and it is good practice for the business world.
6) Have some friends or other business people you can get together with for coffee every now and again and talk about work challenges or just have a laugh. As a freelancer life can be quite isolating and Facebook is no substitute. Find other busy creatives and take a break when you can, it will help immeasurably!