I'm just going to echo what everyone else is saying: Contract. Contract. Contract. A contract isn't some scary legal document full of cryptic legalese. It's a clear description of what services will be provided and what the compensation will be. And on what schedule everything will happen. With details about what happens if things go south. Most client problems are not as cut-and-dry as "he took all of my work and is…
First, to state the obvious which anyone who has been around for some time will tell you a contract is only good if the dollar amount is enough to get a lawyer involved as far as filing and pursuing any legal action. Otherwise good luck with trying to convert a contract into actual dollars because you are right.
Anyway, for the purpose of what you are saying it doesn't have to be a "formal" contract but merely an email trail will suffice to make your point and get the other party (if honorable) to pay. In the past I've even used notes of verbal conversations which I've repeated back the sequence of events to the satisfaction of the other side. (Once again totally depends on who the other side is and their motives.)
Now if they are not honorable, don't have the money, or just want to play games, then if you have enough at stake to hire an attorney a formal contract is a benefit (but not essential since the "trier of fact" as well as the other side will weigh their chance of settling vs. fighting). Consequently, yes, documentation is key but it's more a balance of power rather than the formality of the document).
Unfortunately the article completely ignores the practical aspects of going after someone who owes you money despite what any contract says. As if the law is enough to make the right thing happen. It's not.