Jonathan Clements is Director of Financial Guidance for myFi, a unit of Citigroup Global Markets Inc., and author of The Little Book of Main Street Money, which will be available in May. The views expressed here aren’t necessarily those of Citi or its other employees.

The Wall Street Journal’s Jonathan Clements told readers for years to ignore market trends and keep investing—only to see their portfolios plunge. But in a new book, he says he's sticking to his philosophy.