"Human beings are designed to fail as investors." If James O'Shaughnessy said that in 1998-99 -- he probably did, actually -- the senior managing director at Bear Stearns Asset Management would've been laughed out of most rooms: cocktail parties, daytrading centers, the set of CNBC. But when O'Shaughnessy uttered those words this week at Ibbotson Associates' 2003 Asset Allocation Conference, the University of Chicago classroom -- filled with investment professionals who paid a grand to attend -- nodded approvingly. They were there because, like most Americans, they feel it's high time to revisit asset allocation. Let's be honest: Somewhere over the past five years, a lot of us threw away our asset allocation playbook -- or never bothered to read it in the first place. Let's be even more honest: The financial media -- including my two employers during the past five years, The Wall Street Journal and TheStreet.com -- didn't always champion the cause of asset allocation, even though it is the most important factor for every investor. Consider: A 1991 study by Gary Brinson, Brian Singer, and Gilbert Beebower in the Financial Analysts Journal concluded that asset allocation makes up 91.5% of an investor's return over the course of his lifetime; market timing constitutes about 1.8% and stock selection 4.6% -- yet most investor-oriented publications devote most of their time and energy to short-term, often stock-specific matters that have a negligible bearing on an intelligent long-term investor's portfolio. As investors, we have spent the past three years paying for our failures -- that's what happens when manias end. If we don't fix it now, we will pay for that mistake when we retire. Investors, wake up from your burst bubble-induced torpor: It's time to get your portfolio's asset allocation in position to succeed for the long run. The good news is, proper asset allocation doesn't have to be hard -- it's a lot easier than trying to determine a fair price for General Electric ( GE) or Cisco ( CSCO), it involves less work than market-timing, and it's infinitely wiser than letting it sit unchanged (show of hands: who's still 100% large-cap stocks?). Over the next week or so, I will devote several Long Run columns to asset allocation for the long run. I can say with certainty that a useful asset allocation series is more important -- and, ultimately, more profitable to our readers -- than a year's worth of stories on Cisco, General Electric, and Microsoft ( MSFT). We will discuss specific asset-allocation models in the coming days -- from the simple to the relatively sophisticated. But it's imperative to begin with an overview of the most common pitfalls of asset allocation.