American workers are less prepared for retirement than they were at the beginning of the millennium as the number of companies offering 401(k) plans has declined, according to a study of Census data.

Almost half of employees didn’t have a company-sponsored retirement plan in 2013, compared with 39 percent in 1999, Bloomberg News reports, citing a study by the Schwartz Center for Economic Policy Analysis at the New School for Social Research in New York.

Middle-class professionals and managers are increasingly joining the ranks of low-income people who rely mostly on Social Security, but the average Social Security benefit of $15,700 a year can’t replace the earnings for people with mid-five and six-figure salaries, according to the news service.

Leaving a large company for a smaller one without a benefit plan leaves people more vulnerable.

“The current 401(k) system was designed for a workplace that doesn’t exist for most people: lifetime careers at big corporations that offer benefits,” says Teresa Ghilarducci, an economist at the New School who researches retirement policies. “Saving consistently — which you need to do for just a modest retirement income — isn’t remotely likely.”

Washington, Illinois and Oregon approved laws to create IRAs that would automatically deduct 3 percent from employee paychecks, although none is expected to begin withdrawing money until 2017, Bloomberg reports. Similar laws have been introduced in about 20 other states with Democratic-controlled legislatures, including California and New Jersey.

Jack Bogle, the founder of Vanguard Group, offers four rules to investing for retirement: it’s all about keeping it simple.

Bogle used to have a really basic portfolio that followed an asset allocation known as the 60-40 rule — 60 percent in a U.S. stock index fund and 40 percent in a U.S. bond index fund. But he recently shifted his strategy by a hair: He's now at 50/50, which makes his portfolio slightly more conservative, CNBC reports.

"I just like the idea of having an anchor to the windward," Bogle, who is 86, told CNBC. "I'm not so much worried about having my estate grow."

Here are the 4 secrets he shared with CNBC:

1. Don't rebalance your portfolio
Many investment advisers sell their services in part based on “rebalancing,” or selling your winners to return your portfolio to its asset allocation. Research shows you will reduce your risk of a big loss in the short term in your portfolio with regular rebalancing.
However, there are possible fees, taxes and other pitfalls. "If you want to do it, once a year is probably enough," he said.

2. Don't invest overseas
Bogle keeps his portfolio entirely in U.S. markets. "We have the best investor protections and legal institutions," he said.

3. 'Diversification' means bonds — and bonds only
Bogle uses bonds to offset equity risk in his portfolio. He boosts the bond allocation as he ages, because he wants to reduce the risk of a sudden drop in value.

4. 'Simple' portfolio choices prevents worrying
The genius of Bogle's portfolio, for him, is its simplicity. It's easy for him to track and understand.