A long-term mind-set can put you on the path to mastering the stock market, and it doesn’t get much longer-term than picking stocks to own for decades to come. A lot can change over such a long period of time, but our Motley Fool investors think that Activision Blizzard (NASDAQ: ATVI), Priceline (NASDAQ: PCLN), and Lockheed Martin (NYSE: LMT) have the staying power to deserve a spot in your portfolio. Here’s why.