I usually write full posts on this sort of thing, so I'll just pick one very simple (albeit very important) lesson I've learned the hard way:
You haven't made money until it's in the bank.
You haven't made money until it's in the bank.
I was still in high-school and it was one of my first programming gigs. Got screwed for about U$250, a small fortune for me at that time (IIRC about 3 times the monthly minimum wage in my country).
In a way it was good to have that experience early: it seems that most people don't learn that lesson unless they learn it the hard way.
Then I learned the importance of having written contracts for everything, and then later I learned that contracts can't solve the problem, only mitigate it. You have to select your clients, bill often, etc. But you have do it diplomatically, you can never suggest to your client that you think he'll possibly screw you.
The worst part was in the latter situation I was really expecting to be screwed over but the expected value of the situation still made it worthwhile.
Getting paid on time is surprisingly difficult.
Pay your debts, people remember when you don't.