Reconfiguring your investing strategy for retirement can be tricky. Unless you have the wealth of an oil magnate and can coast by on interest, gearing your portfolio for retirement is really a balancing act of growing investments, preserving capital and generating income.

Someone who retires at age 70 may live to be 90 or even 100 years old. Funding 30 nonworking years will take some cash and likely some risk to keep your portfolio around as long as you might be.

Bankrate asked three financial planners how they would design a portfolio to generate income for a 70-year-old retiree with a nest egg of $500,000 and no other source of income besides Social Security. According to the Social Security Administration, the average monthly benefit for retired workers at the beginning of 2011 was $1,177, which turns out to be an annual income of $14,124.

Balancing income needs with reality

In an ideal scenario, retirees would never need to touch their principal. All of their income needs would be covered by interest payments, capital gains and dividends. But what happens if someone needs more earnings on an annual basis than their portfolio can provide?

In this hypothetical situation, the retiree has income needs totaling $50,000 per year.

That's a problem. For their portfolio to fund their yearly income needs, minus the Social Security payments, they would need to withdraw money at a rate of about 7 percent per year, which means they could come up short if they happen to live a long and healthy life.

"Of course there is no guarantee of the future, but it is reasonable to expect, based on historic data, that they might derive $25,000 a year in portfolio income," says Certified Financial Planner Robert Fragasso, chairman and CEO of Fragasso Financial Advisors in Pittsburgh.

That's assuming annual returns of 7 percent or 8 percent a year and withdrawals of no more than 5 percent.

"To draw off more than 5 percent risks eating principal, and I would not advise them to do that," he says.

There is only one other solution for retirees who cannot reduce their expenses: get a part-time job.

Read on to see how three financial advisers would design a portfolio to generate income from a $500,000 portfolio. All say that at least half the portfolio should be devoted to fixed income, and one recommends a hefty 23 percent allocation to alternative strategies.