In a special episode of his "Mad Money" TV show, Jim Cramer showed investors how to avoid losing money by noting the most common mistakes investors make. He stressed the importance of investing with discipline, using rules to better recognize opportunities and avoid losing money. Cramer said his first rule for investors is to not dig in your heels when you're wrong. Quoting the great economist John Maynard Keynes, Cramer said "when the facts change, I change my mind." Digging in your heels and refusing to acknowledge that your investment thesis is wrong is a sure fire way to lose money, said Cramer. It's natural to be angry, but when the market's turned against you, investors need to adapt.