Image: Mature couple calculating expenses © Abel Mitja Varela, the Agency Collection, Getty Images

"You'll regret it. Maybe not today, maybe not tomorrow, but soon, and for the rest of your life."

Those words, spoken on a tarmac at the finale of the classic film "Casablanca," could be stretched from the intended affairs of the heart to how we plan for retirement.

There are financial decisions we make that seem a good idea at the time, retirement moves we make halfheartedly and needed plans we sweep under the carpet. When it comes to ensuring a lifetime of retirement income, bad decisions may not hurt immediately, but once the pain arrives it can last for decades.

Here are five retirement mistakes that will haunt you, and ways to avoid the haunting entirely:

1. Guesswork, not legwork

How much will you need to retire?

That straightforward question should be a starting point for people of all ages, at all stages of planning. A great number of people, however, do no more than guess at what they should save.

According to the Transamerica Center for Retirement Studies, a nonprofit organization funded by Transamerica Life Insurance, half of workers continue to guess at the amount of money they need to save to feel financially secure when they retire, and a large number (44%) of American workers do not have a strategy to reach their retirement goals.

Of those who do have a strategy, only half have factored in health care costs and one-fifth have factored in long-term care insurance, so their estimates can be inadequate. A reduction or loss of Social Security benefits ranks third in greatest retirement fears.

As part of its research, Allianz Life asked a segment of baby boomers how much money they thought they would need to live on.

"The average number they came up with was $60,000," says Katie Libbe, vice president of Consumer Insights for Allianz Life. "Then we asked them to try to calculate what that means in terms of a portfolio that's going to deliver that for as long as they think they are going to be in retirement. They were at a loss in terms of what they would need.

"They said $500,000 when, if you just apply a 4% withdrawal to that portfolio, they would really need $1.5 million," Libbe said. "They don't know how to do even this basic calculation. You can teach them about some of these individual tactical mistakes, but they can be off even in the strategic sense of how much money needs to be socked away."

Not only is there the danger of not ensuring the longevity of your savings; there is also the temptation of being unreasonably aggressive with your portfolio to make up for not saving as much as you should have. (Are you saving enough for retirement? Use this MSN Money calculator to find out.)

Health care costs also need to be considered. According to research by Allianz Life, the average couple retiring at age 65 will spend approximately $285,000 in health care costs in retirement. As life expectancies continue to increase, baby boomers need to have a plan for covering some portion of managed care for an elderly parent as well as for themselves.

Health care costs, Libbe says, could "eat into a good chunk of their nondiscretionary dollars, which means fewer dollars available for all the fun things they want to do in retirement."

2. 'I'll just keep working'

Most people assume they will retire at a certain age, and many look to salvage inadequate savings by working later into life.

But according to Limra, a global association of insurance and financial services companies, two in five people retire earlier than planned due to a number of factors, including layoffs or illness. For those assuming they will work part time in retirement, many can't due to circumstances beyond their control.

"We did some research recently where we found out that a lot more baby boomers are planning to work longer," Libbe says. "That's basically their backup plan for not having saved enough for retirement. The Limra statistic about two in five people retiring earlier than they expected just shows you can't count on working longer being your plan."

Making the prospect of retiring ahead of schedule all the more precarious is that 70% of respondents to the Transamerica Center survey agreed they could work until age 65 and still not have enough money saved to meet their retirement needs.

"Planning not to retire is simply not a viable retirement strategy," says Catherine Collinson, the president of the center. "Planning to work past age 65 is an important opportunity to continue earning income, save more and help to alleviate a retirement savings shortfall. However, it's important that workers be proactive in setting a retirement savings goal, saving and investing for retirement and having a backup plan if they are forced to retire sooner than expected."

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