Over the past two decades, a relatively conservative portfolio — more stocks than bonds — earned at most 7.5 percent after taxes and expenses. But now, that figure is more likely to be 2.5 percent, warns financial planner Harold Evensky.

The Coral Gables, Fla. planner manages $600 million in a fee-only fund he runs with his partner and wife, Deena Katz.

"Investors who continue to chase strategies or investments without considering this are at great risk — expenses and taxes are going to eat them alive," Evensky tells Barron's in an interview.

"My message: Pay attention to what you can control."

Before, Evensky says, his typical client would hold various types of assets and try to beat the market in each by splitting up the investments. But that can get expensive.

So, instead, now 80 percent of a portfolio is in a "core" account, which is run simply to track global returns at minimal expense and the lowest possible tax rates.

The remaining 20 percent is considered a "satellite" of the core and is invested more aggressively, rotating annually at most, and should beat the market by 2 percent after expenses and taxes.

"To the extent that we try to beat the market, we do that in the remaining 20 percent of the equity portfolio, which is the satellite," Evensky says.

A conservative portfolio, for instance, would contain 40 percent high-quality, laddered fixed-income instruments with maturities of between one and 10 years, while 60 percent would be in equities.

Of that larger slice, nearly half would be considered the core and be invested in broad index funds, tilting toward value stock funds. A third would be in international stock funds.

Meanwhile, the more aggressive — and more expensive — slice would be in six parts of about 2 percent each in a mix of emerging market funds, 130/30-type funds and in funds that invest in real estate investment trusts, high-yield emerging markets and commodities.