When we're staring down a scary and uncertain retirement, it can help to learn from those who've gone before us. If you're freaking out at least a little, wondering whether you're saving enough, and how you can avoid being wiped out by a health-care disaster, you're not alone.

Galloping to our rescue are the folks behind a recent Merrill Lynch Wealth Management survey, which asked for the biggest regrets among Americans with "investable" assets of at least $250,000 -- many of whom have already retired. If we heed their warnings, we can improve our own golden years considerably.

Regret No. 1: Not planning how they wanted to live in retirement
A whopping 38% of the retirees noted this failing. If you haven't clearly envisioned how you're going to live in retirement, you won't be able to estimate how much money you need to save, or what other preparations you may need to make.

Ask yourself whether you plan to downsize into a less expensive home (or town!). Doing so will free up more money for living expenses. Ask whether you plan to do a lot of traveling or any other potentially expensive activities. Consider whether you'll want to work part time in retirement, since that can produce extra income. (If so, decide what kind of work, and begin looking into possibilities well before you retire.)

Your finances also deserve careful thought. Would you like to live off dividend income from blue chips such as Procter & Gamble (NYSE:PG), Pfizer (NYSE:PFE), and IBM (NYSE:IBM)? If so, do your research and add the proper companies to your portfolio.

Regret No. 2: Avoiding a financial advisor
Some 23% of retirees regret not working with a financial advisor earlier. You can find good fee-only advisors via the National Association of Personal Financial Advisors (NAPFA), or by asking your friends for referrals. It's not too late to save your retirement.

Regret No. 3: Choosing luxuries over savings
This regret, voiced by 18% of retirees surveyed, is very telling. Clearly, lots of people found, upon retiring, that they hadn't saved enough. This is a big deal. It means they're forced to compromise and cut back more in retirement than they wanted to.

It's not possible to figure out exactly how much money you'll need in retirement, but if you spend a little time on it, you can come up with a sound estimate. And even if you end up realizing that you're not on track to reach your goals, you can still better your situation significantly. Working a few more years, for example, can add tens or hundreds of thousands to your nest egg. Working seven more years could even net you an additional million.

Regret No. 4: Failure to diversify
Finally, another 18% of respondents cited a lack of diversification as a big regret. Do you have a big chunk of your nest egg invested in your employer's stock? Think twice about that, since you already depend on that company for your salary. Even highly respected stocks can stumble or stagnate for many years, foiling would-be retirees. Coca-Cola (NYSE:KO) has averaged less than 2% growth annually over the past decade. Qualcomm (NASDAQ:QCOM) has lost an average of more than 5% annually during that period.

Is your overall portfolio invested in a wide variety of asset classes? Do you have both large-cap and small-cap stocks? Are you invested abroad, in promising businesses likeBrazil's metals and mining company Vale SA (NASDAQ:VALE) or Finland's Nokia (NYSE:NOK), or in internationally focused mutual funds? Have you added some bonds or bond funds to your mix?

Look closely at your holdings. You may think you're well-diversified with seven different mutual funds -- but perhaps they all focus mainly on domestic large-cap stocks. If so, they may all offer returns that largely track the S&P 500, with little exposure to smaller or more global fare.

Heed their advice
The retirees surveyed offered specific guidance for those of us within 10 or 15 years of retirement:

  • Decide what is most important to you in retirement (51%).
  • Have a plan to manage income throughout retirement (47%).
  • Pay down debt (40%).
  • Account for unexpected costs and risks (38%).
  • Pursue home ownership (24%).
  • Be cautious of taking investment risks (21%).

Take action now to avoid having major regrets like these down the road. If you'd like a little extra help, try our Rule Your Retirement newsletter. You can check out all past issues, filed with practical advice, stock and fund recommendations, and model portfolios, free for 30 days.

Longtime Fool contributor Selena Maranjian owns shares of Coca-Cola and Procter & Gamble. Coca-Cola, Nokia, and Pfizer are Motley Fool Inside Value recommendations. Coca-Cola and Procter & Gamble are Motley Fool Income Investor recommendations. The Fool owns shares of Procter & Gamble. The Motley Fool is Fools writing for Fools.