What to Do Instead: The simplest way to tether your withdrawal rate to your portfolio's performance is to withdraw a fixed percentage, versus a fixed dollar amount adjusted for inflation, year in and year out. That's intuitively appealing, but this approach may lead to more radical swings in spending than is desirable for many retirees. It's possible to find a more comfortable middle ground by using a fixed percentage rate as a baseline but bounding those withdrawals with a "ceiling" and "floor."

Mistake 2: Not Building In a 'Fudge Factor'

Another drawback to employing a fixed-dollar withdrawal method--especially if the viability of your plan revolves around a fixed annual dollar amount that's too low--is that it won't account for the fact that your actual expenses are likely to vary from one year to the next. Try as you might to anticipate them, discretionary expenditures like travel or new-car purchases, or unplanned outlays for home repairs or medical expenses, have the potential to throw your planned withdrawal rate off track. If you calibrate your anticipated spending based on your basic monthly outlay alone--groceries and utilities, your property-tax bill, and so forth--and don't leave room for these periodic unplanned expenses, your actual spending rate in most years is apt to run higher than your planned outlay. In short, a withdrawal plan that looked sustainable on paper actually may not be.

What to Do Instead: Smart retirement planning means forecasting not just your regular budget items but those lumpy outlays, too, whether special travel plans or new-car purchases. In addition to building those extraneous items into your budget, it's also wise to add a "fudge factor" in case those unplanned outlays exceed your forecasts. How much padding to add depends on both how specific you have been in forecasting your expenses (the more specific and forward-looking your forecasts, the less of a fudge factor you'd need to add) as well as how conservative you are (if you don't want even the slightest chance of running out of money, you'll need to add more padding/assume a higher spending rate). Armed with that more-accurate depiction of your anticipated spending, you can then test the viability of your withdrawal rate.

Mistake 3: Not Adjusting With Your Time Horizon

Taking a fixed amount from a portfolio--whether you're using a fixed dollar amount or a fixed percentage rate--also neglects the fact that, as you age, you can safely take more from your portfolio than you could when you were younger. (That assumes, of course, that you're planning to spend most of your portfolio and are not planning to leave behind large sums for your heirs or for charity.) The original "4%" research assumed a 30-year time horizon, but retirees with shorter time horizons (life expectancies) of 10 to 15 years can reasonably take higher amounts.

What to Do Instead: To help factor in the role of life expectancy, David Blanchett, Morningstar Investment Management's head of retirement research.

Mistake 4: Not Adjusting Based on Your Portfolio Mix

Many retirees take withdrawal-rate guidance, such as the 4% guideline, and run with it, without stopping to assess whether their situations fit with the profile underpinning that guidance. The 4% guideline, for example, assumed a retiree had a balanced stock/bond portfolio. But retirees with more-conservative portfolio mixes should use a more-conservative (lower) figure, whereas those with more-aggressive asset allocations might reasonably take a higher amount.

What to Do Instead: Be sure to customize your withdrawal rate based on your own factors, including your portfolio mix. Of course, a financial advisor can also help you create a customized spending target.

Mistake 5: Staying Wedded to Your Portfolio's Income Payout

Many retirees operate with the assumption that they can spend whatever income distributions their portfolios kick off--no more, no less. As yields on safe securities like CDs and short-term bonds have shrunk over the past several decades, they've had to make do with less or have ventured into higher-yielding securities with higher risk. They assume that as long as they spend only their portfolio's income distributions, their retirement plans will always be safe. However, the distinction between income distributions and principal withdrawals is an artificial one, as discussed here; whether your withdrawal comes from income or withdrawal of capital, it all counts as a withdrawal.

What to Do Instead: While there's no one single "right" way to manage a portfolio to deliver your spending needs in retirement, it's wise to have a plan. Will your withdrawal come from income distributions, periodic withdrawals of capital (through rebalancing, for example), or a combination of the two? The method that I favor is building a portfolio with an emphasis on long-term total return; retirees can then see how far any income distributions from that portfolio take them, and then use rebalancing proceeds to help make up for the rest.