Applying Modern Portfolio Theory is the hard part. Right now, for instance, that would mean snapping up emerging-markets stocks, commodities, and government bonds, assets that completely fell apart in 2013. Worse yet, you’d have to pay for these losers by selling shares of last year’s big winners, like the SPRDR S&P500 ETF, that soared about 30% higher.

With such wide divergences in the performance of different assets in the last year, it’s even harder than usual for investors to find their courage and do the right thing, says Mitch Tuchman, CEO of the Web-based retirement advisory MarketRiders.

But that’s the point, says Tuchman: None of us is smart enough to pick the winners and avoid the losers every year, so hedge your bets with measured exposure across all asset classes. To keep that exposure consistent, investments that have grown must be trimmed periodically and the profits shifted to assets that every impulse in your limbic lobe tells you to run from. The bigger the gaps in performance, the more calls Tuchman gets from subscribers begging him to move all their money into or out of an asset class.

“We’re almost always able to talk them off the ledge by showing them performance studies,” he says. “But Modern Portfolio Theory remains a counterintuitive concept for most investors, which is why we started Rebalance”.

That follow-on service takes trigger-pulling duties out of the hands of subscribers who let Rebalance oversee portfolios domiciled at either Charles Schwab or Fidelity Investments. Adjustments are supervised by a team that includes authors and experts such as Burton Malkiel (A Random Walk Down Wall Street) and Charles Ellis (Winning the Loser’s Game) as well as experienced hands such as Jay Vivian, a former managing director of IBM’s retirement program. A passive investing approach using low-cost ETFs exclusively means most portfolios are rebalanced only a couple times a year for an annual fee of 0.70% of assets plus trading costs. Any savings from trading large amounts get passed on to subscribers.

If a subscriber has chosen an aggressive approach, he or she could have to make quite a few trades affecting just a handful of shares several times a year. However, only about half of the subscribers to the $150-a-year MarketRiders service regularly rebalance, says Tuchman.

To ease the burden, MarketRiders has just added a series of simplified asset allocation templates for so-called Lazy Portfolios. They may contain as few as two or three ETFs in a simple 60%-40% equitybond allocation, and they rarely contain more than 10. The most popular of these are regularly tracked by Paul B. Farrell, a columnist for Barron’s sister publication MarketWatch. He wrote the book on Lazy Portfolios—The Lazy Person’s Guide to Investing.

Some Lazy Portfolio advocates like Farrell advise rebalancing only when new money is added. MarketRiders competitor MyPlanIQ is at the other end of that spectrum, advocating monthly “tactical” rotation out of asset classes losing momentum and into those gaining momentum. Having just completed a Website facelift, the service also is doubling its annual fee. But MyPlanIQ subscribers who sign up or re-up before March 15 can keep its $100 annual price tag for a basic subscription.

New portfolio site PJMint uses quantitative trading algorithms that trigger even more frequent rebalancing. PJMint’s alarms tell subscribers when to move out of a deteriorating asset class and into cash or, perhaps, a better-performing asset. PJMint charges from 1%-to-2% of assets under management annually, depending on the plan.

There’s broad consensus that MPT is a good and necessary portfolio discipline, just not much agreement on which flavor works best.