John Harrington writes the excellent blog Photo Business News & Forum. He usually has simple and clear advice on how to ensure that you can earn a living as a freelance photographer. Much of his legal advice is US centric, but I think all other stuff applies across the board. Whether it is how to word contracts, deal with clients or protect your own photographic interests.
From his recent list of facts:
"Fact #1: If every time you produce images, the copyright to them is not yours, you will not earn money - any money - from them in the future. "
"Fact #6: Just because a client says they won't pay for something, doesn't mean you must accept, and work under, those terms. You have the power to say "no"."
There are some between and after these that are useful reading for those who wish to hit the next level. And as he points out, even if you don't want to go pro and want to continue doing photography on the side, to do it responsibly you should still follow these rules.
As a commenter points out:
"One could say, "In a free market, shouldn't I be free to undercut anyone and compete on price?" Certainly, but in a free market, enlightened self interest makes the whole concert play, and you are really undercutting yourself in doing so. Why is this reality so hard for some to understand?"