The first decade of the 21st century, and the second one that’s drawing to a close, have reinforced for investors some timeless market lessons: Returns can vary sharply from one period to another. Holding a broadly diversified portfolio can help smooth out the swings. And focusing on known drivers of higher expected returns can increase the potential for long-term success. Having a sound strategy built on those principles—and sticking to it through good times and bad—can be a rewarding investment approach.