NEW YORK (MainStreet) — The message finally seems to be getting through: Americans are getting serious about saving for retirement – and some tangible results prove it. A soaring stock market for the past few years hasn't hurt either. Fidelity Investments reports the number of "401(k) millionaires" has nearly doubled in the past two years. About 72,000 retirement accounts held with the giant brokerage firm have balances that exceed the magic number, and while it's just a fraction of the 13 million plan participants that the firm administers, the number has nearly doubled since 2012.

Most of us have heard the usual advice: defer enough to max out your employer's matching contribution (if offered), make sure your investments are well-set without taking a bunch of unwise bets and kick-up your contributions whenever you get a raise. But is all of that enough to build a seven-figure balance for retirement?

Considering that the average 401(k) balance at the end of 2014 was just $91,300, and that's a record high, the big-money savers seem far ahead of the game. More averages to consider: the typical annual worker contribution was just under $10,000 and the average savings rate was 8.1% -- the highest since year-end 2011, according to Fidelity.

The keys for success as a 401(k) millionaire seem to be 1) Time in the market, 2) Super-sized savings and 3) A 'leave it alone' approach.

Fidelity researched the most common attributes of the seven-figure savers and found:

  • The average 401(k) millionaire was 59 years old and had been with his company for more than 30 years. But that was yesterday, and this is today; times have changed. The key: start early and hang in there. When you change jobs, resist taking a distribution – roll your present balance into your new employer's plan or just keep it where it is. Just don't cash it out. And resist loans, too.
  • Most of the workers with the million-dollar retirement plans made less than $150,000 but they contributed more than the average saver, putting back about 14% of their income. Combined with the average company match of 5% that meant they were saving about 20% annually.
  • And these workers who were well-set for retirement had portfolios that contained a broad mix of investments, including a healthy serving of stock mutual funds. Now, that doesn't mean being too aggressive but you don't want to be too conservative, either. "Historical data suggests that a diversified portfolio of stocks can deliver higher returns than bonds or other fixed income investments over time," the Fidelity report says. "This lesson was not lost on our 401(k) millionaires, who had an average of 75% of their assets in company stock and stock mutual funds and achieved a median annualized return of 4.8% in their 401(k) over the 12-year period of our study (2000–2012)." And that included some tough times for stocks.

Of course, not everyone can amass a million-dollar nest egg, but not everyone requires that much to live comfortably in retirement. As a wise man once said, "Aim high, and you probably won't shoot yourself in the foot."

--Hal M. Bundrick is a Certified Financial Planner and contributor to MainStreet. Follow him on Twitter: @HalMBundrick