[dropcap]U[/dropcap]nless you live near an amusement park that does a lot of advertising, you probably didn’t know that Aug. 16 is National Roller Coaster Day. Actual roller coasters provide people with thrills. But as an investor, how can you stay calm on the “roller coaster” of the financial markets?

Here are some suggestions:

Know what’s in front of you. If you’ve ever ridden a roller coaster in the dark, you may find it scarier than if you boarded it in daylight &...