Even as the financial system collapsed last year, and millions of investors lost billions of dollars, one unlikely investor was racking up historic profits: John Paulson, a hedge-fund manager in New York.

His firm made $20 billion between 2007 and early 2009 by betting against the housing market and big financial companies. Mr. Paulson's personal cut would amount to nearly $4 billion, or more than $10 million a day. That was more than the 2007 earnings of J.K. Rowling, Oprah Winfrey and Tiger Woods combined.

How did he do it? Believing that a housing-market collapse was coming, Mr. Paulson spent over $1 billion in 2006 to buy insurance on what he then saw as risky mortgage investments. When the housing market cracked and the mortgages tumbled, the value of Mr. Paulson's insurance soared. One of his funds rose more than 500% that year. Then in 2008, he shorted financial shares, or wagered that they would fall in price, profiting again when these companies collapsed.

And are there any investing skills that average investors can learn from his success? Yes. There are no guarantees, of course, but the success of Mr. Paulson and a few other underdog investors lends encouragement to individuals trying to compete with Wall Street's pros.

Here are eight investing lessons of Mr. Paulson's $20 billion gamble, the greatest trade in financial history: