The S&P 500 (^GSPC -0.14%) has soared higher over the past few months, putting the famous stock market index up 18% over the past year. At approximately 20.4 times expected earnings, the S&P 500 has become notably more expensive than its 30-year average of 17.2. It’s a legitimate cause for concern; the price you pay for stocks always matters.

However, it’s not as simple as avoiding the stock market until prices come down. Expensive markets don’t always fall. Meanwhile, trading in and out requires constantly guessing correctly about something that nobody can predict: what the market might actually do next. History shows that investors are better off simply investing as much as possible in the S&P 500 for as long as possible, riding the…