When the stock market is in a funk, as it is now, down 25% so far this year and firmly in bear market territory, it's easy to forget the power of a simple buy-and-hold investing strategy. Because every steep correction is invariably followed by a bull market, one that tends to go far higher than the previous peak before the downturn hit, those valleys tend to smooth out until they hardly even register on a stock chart. Bear markets are also typically measured in months, while bull markets are measured in years.