There's a saying on Wall Street: "Bull markets don't die of old age."

But some investors worry that the bull market in U.S. stocks will end sooner than later.

Worried? Consider these investing tips.

Don't try to time the market. While it's painful to ride out hard times or invest when stock prices are near record high levels, take heart knowing that the stock market's long-term average return is 10 percent annually for buy-and-hold investors. If you want to do something, consider tweaks to your investment strategy — buying alternative assets, foreign stocks or different kinds of assets within the U.S. market.

Stay in the market. "You don't want to be out of the market entirely, but if you don't like the pricing of stocks right now, there's nothing wrong with holding cash until you do," said Brad McMillan, chief investment officer at Commonwealth Financial Network.

Open a money-market account that pays an attractive rate (several banks offer 1.75 percent APY or higher) and invest in the market over time, McMillan said. This strategy, known as dollar-cost averaging, helps ensure you don't invest all of your money at the market's peak.

Diversify, diversify, diversify. A well-diversified portfolio should include a mix of different assets (including stocks and bonds), and diversification can be especially comforting when the stock market is on shaky ground.

If stock prices drop and stay down, expect slower economic growth, lower earnings and the Federal Reserve to lower interest rates. All that runs counter to what is currently happening. Central bankers are expected to raise rates two more times this year.

Even if you don't foresee a full-fledged bear market on the horizon, McMillan said to consider these three strategies:

Add more bonds. Bonds are appealing because they can help balance potential losses from stock investments.

Buy dividend-paying stocks. When stock prices aren't skyrocketing higher anymore, then dividends will generate more of your total return. Utility companies and real estate investment trusts (REITs) historically pay high dividends and have steady sources of revenue, even during economic slowdowns.

Buy noncyclical companies. Similarly, the consumer-staples sector — companies that make food or household products, for example — has a more reliable source of revenue.

Consumers will cut back on clothing purchases before they will stop buying groceries.

Anna-Louise Jackson is a writer at NerdWallet. E-mail: ajackson@nerdwallet.com. Twitter: @aljax7.