The Vanguard S&P 500 ETF (NYSEMKT: VOO) is one of the best ways to invest in the S&P 500, which has been a pretty smart strategy over the long term. Since 1965, the S&P 500 has produced a total return of 10.2% annualized. The Vanguard ETF has an expense ratio of just 0.03%, so you get to keep most of your gains.While there's no guarantee that the S&P 500 will achieve the same level of performance.