Over the past five years, stocks have only gained once in September.No matter what the market is doing, it's a great idea to keep an eye on the moves of Warren Buffett. He's proved his understanding of stocks over time and his ability to translate that into investing success. The top investor has sat at the helm of Berkshire Hathaway for nearly 60 years, and over that time period has helped drive a compounded annual return of about 20%. That far outshines the S&P 500 (^GSPC -0.32%), which delivered a 10% su ...