Over the past decade, financial experts have been busy figuring out how much retirees can safely spend each year without running out of money. The result? We now have a better grasp of the risks involved -- and a pretty good idea of what a sensible strategy looks like.

There's just one problem: Sensible won't cut it for most Americans.

Wrong answers. Spending down a portfolio in retirement is a treacherous business, because you don't know how long you will live, what the inflation rate will be or how financial markets will fare.

Faced with all this uncertainty, experts typically suggest two solutions. First, you might limit your initial portfolio withdrawal rate to just 3% or 4%, equal to $3,000 or $4,000 for every $100,000 saved. This is well below the 5% and 6% withdrawal rates that used to be advocated and reflects, in part, a concern about today's lofty stock valuations and low after-inflation bond yields.