Instead it should be built around getting the most out of the factors you can control. Fund expenses, for example. If Arnott is right, stocks would beat bonds by about a percentage point, so a fund charging 1.5% in annual expenses is on its way to losing the race before its manager makes his first trade. A solid index fund charges a tiny fraction as much.

You can also make sure to minimize taxes. If you have money in taxable accounts, take a serious look at funds that are managed for optimal tax efficiency. Once you've maxed out the match on your 401(k), consider sheltering money in a Roth IRA if you are eligible. You might also convert some of your investments in a traditional IRA to a Roth. With a Roth, you pay your taxes up front; the optimal amount to put in will depend on your current tax bracket and what you expect your future tax bracket will be.

But with tax rates near a historical low today even as the government's future obligations keep piling up, there's a solid case for paying at least some taxes ASAP.

"Draw a chart of tax rates - if that was a stock, you'd buy it now," says Robert Gordon of Twenty-First Securities in New York City.

But at the top the list of things you can control is how much you save. You need to be aggressive here. That means saving at least 10% of your income, and 15% to 20% if you can possibly swing it, especially if you've gotten off to a late start.