Some things I've learned the last 5 years:

• Get yourself a ironclad set of contracts and use them. Every time.
• Don't try to "network" excessively, but be sure to make some friends.
• Don't take on too much work.
• Never miss a deadline.
• Make sure every project gets the attention and care it deserves.
• Don't freak out if you don't know what your next project is going to be.
• Find yourself a couple of people you enjoy working with and stick with them – you can't do everything by yourself.
• Be humble (I'm not, but I'm an asshole).
• Don't chase the easy solutions, it'll make your work boring.
• What you have in your portfolio is what you get hired for, so pick carefully.
• Always stay hungry and never give up.