The wisdom proffered by most financial advisers is to take out no more than 4 percent of savings in the first year, then increase that amount annually by the inflation rate. If you have $100,000 in savings, you can take out $4,000 the first year ($333 a month) and $4,120 the second year.

Professor Spitzer, who studied various theories of calculating withdrawals of retirement income, recommends that retirees who face the reality of an uncertain economy should recalculate withdrawals from their retirement accounts at regular intervals. He suggests limiting withdrawals to 4 percent for the first five years, with no increase for inflation, and then recalculating. Even if the total amount has declined, the number of years that you'll need money also will have decreased. "But if the market comes back with a vengeance," Spitzer says, you may find yourself mathematically able to give yourself a significant raise.

In the meantime, continuing to work at least part time can make up the shortfall.

But don't make employment your only retirement plan. In a retirement confidence study, the Society of Actuaries determined that 40 percent of people retire earlier than they planned because of their health or because they just can't find employment.