From his July 2003 Memo "The Most Important Thing":
The ingredients in defensive investing include (a) insistence on solid, identifiable value at a bargain price, (b) diversification rather than concentration, and (c) avoidance of reliance on macro-forecasts and market timing.
“Defensive investing” sounds very erudite, but I can simplify it: Invest scared! Worry about the possibility of loss. Worry that there’s something you don’t know. Worry that you can make high quality decisions but still be hit by bad luck or surprise events. Investing scared will prevent hubris; will keep your guard up and your mental adrenaline flowing; will make you insist on adequate margin of safety; and will increase the chances that your portfolio is prepared for things going wrong. And if nothing does go wrong, surely the winners will take care of themselves.