1) Loss cutting: Trading has this amazing historical footnote: If you study the great traders throughout history, they all share the same statement as their number one rule: CUT YOUR LOSSES! Capital preservation “keeps you in the game.” It is especially important once you understand the math: a 25% drawdown requires a 33% gain to get to break even; Down 33% means you need to rally 50% to get back to square one; As we saw in 2008-08, a -50% loss requires a +100% gain to get back to even. In sports “Defense Wins Championships.” The same goes for stock trading. Most traders need to focus more on defense.
Even Warren Buffett understand the traders credo: “The first rule of investing is don’t lose money. The second rule is don’t forget Rule No. 1.
---The rest from Ritholtz (source)