Buffett On Investing:

Don’t be too fixated on daily moves in the stock market (from Berkshire letter published in 2014)
Don’t get excited about your investment gains when the market is climbing (1996)
Don’t be distracted by macroeconomic forecasts (2004)
Don’t limit yourself to just one industry (2008)
Don’t get taken by formulas (2009)
Don’t be short on cash when you need it most (2010)
Don’t wager against the U.S. and its economic potential (2015)

Buffett On Management:

Don’t beat yourself up over wrong decisions; take responsibility for them (2001)
Don’t have mandatory retirement ages (1992)
“Don’t ask the barber whether you need a haircut” because the answer will be what’s best for the man with the scissors (1983)
Don’t dawdle (2006)
Don’t interfere with great managers (1994)
Don’t succumb to the attitudes that undermine businesses (2015)
Don’t be greedy about compensation, if you’re my successor (2015)

Source: Here’s What Buffett Wouldn’t Do, and Maybe You Shouldn’t Either