3 low-stress ways to invest for retirement

Keeping on top of your finances is no simple feat. But investing your retirement savings can be surprisingly easy. Check out these three smart, low-stress ways to save for the future. Pick just one and you'll have done a good deed for your post-workaday self.

A target-date fund is a mutual fund that invests in other mutual funds so you don't have to. It's a one-stop shop. Because a target-date fund is investing in other mutual funds, it provides all the diversification that you need.

In fact, all the things you're supposed to do as an investor — such as making sure your investments are diversified and rebalancing your account — is all taken care of for you.

How to invest in one: If you have a 401(k) or other retirement plan at work, there's a good chance you have access to a target-date fund. About two-thirds of 401(k)s offer one, according to the Investment Company Institute.

If you don't have a workplace plan, then it probably makes sense to open an IRA. You can do that quickly and easily at an online brokerage, and you'll be able to invest in a target-date fund there.

Things to know: Pick the right target-date fund for you. The "date" in its name should match your expected retirement year. It's OK to estimate. The target-date fund is set up to get more conservative over time, as you near retirement. Once you actually get close to retirement — like, five or 10 years out — you should check in with all of your investments to make sure they suit your situation.

Also, be mindful of fees. Some 401(k)s offer access to inexpensive mutual funds, but many don't. How high is too high? If you're paying an expense ratio of 0.66 percent, that's average — and kind of pricey. (Consider that the target-date funds offered by Vanguard Group have an average cost of 0.13 percent.) Anything over 1 percent is definitely too high.

And "broad-based" is simply a mutual fund that spans a long list of companies, 3,000 to 4,000 of them potentially, depending on the index. But no less an investor than Warren Buffett has said that most investors will do fine by consistently buying a mutual fund that tracks the S&P 500 index, which covers 500 large, publicly traded U.S. companies.

How to invest in one: Ideally, your workplace retirement plan offers a broad-based index fund. If that's not the case, but your employer does offer a match, then find a low-cost mutual fund in your workplace plan, put enough money in to get that match, and then think about devoting the rest of your retirement-savings dollars to an IRA or Roth IRA.

Things to know: The stock market is volatile. It rises and it falls. Crashes, sometimes. If you don't panic, and if you keep your money in, it's highly likely you'll make money over the long term. But if watching your account temporarily plunge like an out of control roller coaster is going to make you want to abandon stocks entirely, then you need to balance your investments with a bond mutual fund. That's because stocks provide the potential for long-term growth, while bonds offer lower returns but lower volatility, too. Keep things simple with a "total bond market" fund.

And what I said above, about target-date-fund fees? Don't pay anything close to 1 percent.

A robo-advisor is a financial advisory firm that uses computer technology to simplify the investing process. Generally, a robo-advisor creates a handful of investment portfolios and picks the best one for an individual investor based on that person's financial goals and tolerance for risk.

How to invest in one: Generally, investing with a robo-advisor is entirely separate from investing in a 401(k) or other workplace plan. Most robos offer IRAs (Roth and traditional) for retirement investing.

To invest with a robo-advisor, you simply have to pick one and fill out its questionnaire. Just like a live financial adviser, robos need to ask questions to make sure they're investing your money in a way that's appropriate for your situation and goals. Finally, link an account or roll money over.

Things to know: When choosing a robo-advisor, make sure the company offers the account type you need — say, a Roth IRA — and has an account minimum that fits your needs.