It s no secret that most Americans aren t saving enough for retirement the latest figures cite a shortfall of $6.6 trillion. But with a strategy to avoid the most common retirement planning mistakes, aspiring retirees can close that gap.

The mistakes you make along the way can really shrink the size of your retirement nest egg down the road," says Mark Miller, a syndicated retirement columnist and author of The Hard Times Guide to Retirement Security.

First on the list: Many savers still rely on the old adage that you ll need 70% of your pre-retirement income to fund your lifestyle in your golden years.

The biggest pothole in this logic is that it assumes your home is paid for in retirement... but today many retirees still have mortgages, says Manisha Thakor, a Houston-based CFA and the author of On My Own Two Feet, a personal finance guide for women. And that figure doesn t take into account the rising cost of health care or, say, a newfound desire to travel extensively post-retirement.

Here are four more costly retirement mistakes that could be eating away at your savings.

1. Underestimating health-care costs

Most people know that Medicare isn t enough to cover healthcare costs in retirement. So how much do you really need? More than you think. In 2009, a typical 65 year-old male retiree could need as much as $378,000 in savings to cover health insurance premiums and out-of-pocket health expenses in retirement, says the Employee Benefit Research Institute. A typical woman of the same age would need as much as $450,000.

Strategy: It's important to stay on top of the latest healthcare legislation, experts say. Your medical history matters too do you have a history of chronic illness, or does breast cancer run in your family? A financial planner can help you estimate costs as well.

2. Giving too much money to adult children

Four out of five parents regularly give money to their adult kids, according to a 2009 study published in the Journal of Marriage and Family typically, nearly once a month. That adds up. And a survey by CreditCards.com found that 42% of parents had paid off a debt for an adult child, with auto loans, medical expenses and utilities being the most common.

Strategy: Like any other expense, gift-giving belongs in your budget now and post-retirement. Saving for your retirement is the priority, says Bill Winterberg, a CFP and principal at FP Pad. Giving to kids has to come second. He also recommends that even parents who are on track for retirement create a written policy that dictates gift-giving objectives, which will help them decide what to fund and how much. Think about the kinds of gifts you might want to or feel obligated to make (i.e. paying off an adult child s debt), and try to encourage positive activities with your gifts, like higher education, Winterberg says.

Over the last few years, the stock market has spooked investors, especially younger ones. In 2009, only 22% of people younger than 35 said they were willing to take above-average investment risk, compared to 30% a decade earlier, according to a survey by the Investment Company Institute. That s the largest drop of any age group, though the trend toward avoiding the stock market has persisted across ages.

But being overly cautious can be costly in the long-term. Over long time periods, stocks have outperformed bonds: The average annual gain of the S&P 500 from 1926 to 2010 was 9.8% but only 5.4% for long-term U.S. Treasuries. Even though it s scary right now, there s a good chance a healthy stake in equities belongs in your portfolio.

Strategy: Consult a financial advisor about your investment strategy or use these tools to figure out an appropriate mix of assets. Wes Moss, the chief investment strategist at Capital Investment Advisors in Atlanta, advises you to own your age in bonds, meaning that a 50-year-old would have about 50% give or take 10% or so of his portfolio in bonds with the rest in various equities. Click here to get the latest on where to invest now. As always, fees matter: A recent Morningstar study showed that fees are the single biggest predictor of overall profitability of a mutual fund. Make sure you aren t paying more than about 1% annually, Moss says.

4. Thinking you ll enjoy all that free time

What happens if the dream of lounging on the porch and reading three papers a day turns out to be a nightmare? Most people are social creatures, says Thakor. An unstructured retirement can lead to everything from overspending -- so many hours to fill, so many places to shop -- to depression, she says. Or perhaps this is the moment to play golf every day, or see the world activities that you can afford, as long as you account for them and budget in advance.

Strategy: Quiz your retired friends. What are they doing with their free time, and how much might that add to your income needs? When you review your portfolio and savings rate, recalculate how much you will need to sock away for retirement, factoring in these extras; you might find you need to kick in an extra percentage point to your 401(k) beyond what you anticipated to cover new hobbies. These tools can help.