What would be the single piece of investing advice you would give yourself if you could travel back in time?

Question is in the title. Please try to avoid answers like, "Buy Apple when the company goes public", or "Avoid Enron". I am looking for more generic, anecdotal, types of advice/tips that hold true over time from your own personal experience. I am only about one year into investing, and I will be turning 31 come the end of the month. $AMC Entertainment(AMC.US)$ $SPDR S&P 500 ETF(SPY.US)$ $Nasdaq Composite Index(.IXIC.US)$ $Dow Jones Industrial Average(.DJI.US)$