After more or less a flat year in 2015, the broad U.S. stock market is down, as of mid-March in 2016. But that's no reason to throw up your hands in despair. Instead, this is a time to remember that disciplined investing is what matters most. It's easy to stay invested when the market's going great guns. What takes discipline is staying the course when things get rough—instead of making rash moves that may actually cut into your long-term investment returns.