If the key to successful investing is buying low, then an abundance of caution is in order if you're rebalancing your portfolio.

This year's rally has sent prices for everything from stocks to bonds to gold so high that easy pickings are unusually hard to find heading into 2010.

"There isn't that much to be excited about," says Ben Inker, asset allocation director at GMO, a Boston-based manager of $102 billion for institutional clients such as endowments, foundations and pension funds.

If pros like Inker admit difficulty finding decent investments, it's especially hard for average investors looking to adjust their mix of stocks, bonds and other assets at year-end. After all, many mistakenly buy when a rally has nearly run its course, or sell in panic near the bottom.

Sure, the market feels safer than it did when almost everything was crashing _ to prices so low it seemed there was nowhere to go but up if you were patient.

Those with the fortitude to take money off the sidelines last winter or spring have been richly rewarded. The Standard & Poor's 500 is up more than 60 percent since bottoming out March 9. A broad measure of the bond market, the Barclays Capital U.S. Aggregate Bond Index, is up more than 7 percent since then. Riskier high-yield bonds, meanwhile, have surged more than 60 percent.

Government policy that's made it easier to borrow has helped fuel some of the gains for more speculative investments, from junk bonds to racy emerging markets stocks. That policy won't be ending anytime soon _ Federal Reserve Chairman Ben Bernanke pledged Monday to maintain interest rates at near zero for an "extended period" to boost spending.

"It's going to be tougher in 2010, because we've had this big flow into risky assets," Inker says. "They were all cheap last winter, but they've all gone up 60 percent-plus."

Even after those gains, nearly everyone agrees the economic recovery is still shaky. So strategists like Inker are having a harder time figuring out where to put money to work.

"It's a little bit frustrating, because the temptation is to say, 'Well, stocks have had a great run, so let's take some money and put it back into fixed income,'" Inker says. "But then you look at the risks in bonds, you say, 'It's not clear what we really want to own.'"

Inker and another asset allocation expert, Rob Arnott, both worry about long-term inflation eroding bond returns. And Arnott _ chairman of Newport Beach, Calif.-based Research Affiliates, and manager of the $14 billion Pimco All Asset Fund _ doesn't like much in the stock market, either. He contends it's "expensive, and priced as if the recession is over."