Warren Buffett's stock keeps rising. Not just literally—he's increased his net worth over the past month—but figuratively as well: Both Barack Obama and John McCain now cite him as an authority on the campaign trail. So pay attention to his op-ed in the Times today, sharing with investors his golden rule: "Be fearful when others are greedy, and be greedy when others are fearful." Short-term turmoil will not change the fact that, in the long-run, the markets grow, and so Buffett advises investors to follow his lead and take advantage of the cheap prices of American stocks. "Bad news is an investor's best friend," he writes. "It lets you buy a slice of America's future at a marked-down price."