A few key rules of thumb that have worked very well for me are:
1) Keep your 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 who will lose is you. 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 audiobooks, 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 easel, drawing board, or computer for the hours it will take to complete the job, and do that!
4) I highly recommend that you don't do spec work. Your time and talent have value, and people often only value you as much as they are paying you; sad but true. Stick to your rates and do equally good work for everyone. That being said, occasionally you may want to donate something for fundraising efforts. In that case, keep the upper hand. Artists: you choose which causes will receive your donations and how frequently; designers and illustrators: make sure you retain complete creative control of your aspect of the project, and that others on the project are volunteering their time also.
5) Put your title 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. If you have a website or social media where people can see your work, include the link.
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. Find other busy creatives and take a break when you can, it will help immeasurably!