NEW YORK (CNN/Money) - I'm rolling over $15,000 from an old 401(k) into an IRA, and I don't want to take a lot of risk since the value of my account is already down 50 percent from its high. I'd be content to earn 3 percent for the next 15 years as long as I don't lose any more money. What do you suggest?

-- Paul Moore, Largo, Florida

If merely earning 3 percent over the next 15 years or so with safety of principal were your only consideration, you could easily achieve that goal by buying U.S. Treasury bonds.

Since they're not issued new in 15-year denominations, you'd have to buy a 20- or 30-year bond with 15 years left on its term, which means you'd have to incur transaction costs that would cut into your yield.

Still, with Treasury bonds in the 15-year-maturity range yielding 4.5 percent or so recently, you should be able to come away with 3 percent a year quite easily, assuming you hold the bond until it matures.

But is that really the way you want to be investing long-term retirement savings, battening down the hatches in bonds? That approach might be okay if you've got so much money tucked away for retirement in other places that it doesn't matter whether your fifteen grand grows very much.

But if you're like most of us, you probably really can't afford to have such modest return goals. For most of us to have a decent shot at maintaining anything close to our current standard of living in retirement, we need to get a bit more oomph out of our savings.

We need some long-term growth to pump up our savings, to bolster the size of our retirement nest egg.

As I point out in my new book, "We're Not In Kansas Anymore: Strategies for Retiring Rich in a Totally Changed World", most people saving and investing for retirement (and I suspect that includes you) need to have a portfolio that offers a bit of protection against short-term swings in the market, but also provides a shot at some capital growth.

And that means divvying up our money between stocks and bonds, and perhaps throwing in a little cash, i.e., money market funds and the like.

Of course, once you introduce the idea of stocks, you automatically do away with any notion of return guarantees or minimums. Stocks fluctuate in value, sometimes wildly, no two ways about it. But by managing your exposure to stocks and including less volatile assets like bonds and cash, you can at least mitigate stocks' volatility while increasing your potential for return.

Besides, when you're investing for retirement, it's not the ups and downs your portfolio experiences today that should be your concern. It's how large the portfolio will be when you eventually retire.

If you manage to sidestep all the fluctuations in the market, but that strategy leaves you with an anemic nest egg, have you really helped yourself? No.

You're much better off charting a course that allows for some short-term yo-yoing in the value of your savings, but that has a better chance (although no guarantee) of doubling or tripling the value of your savings over 15 years. It's called taking prudent risks.

So instead of buying and holding Treasury bonds (the actual market value of which, by the way, will fluctuate as interest rates rise and fall) or sitting out the next 15 years in low-risk investments like money-market funds or CDs, I recommend you create a diversified portfolio with your fifteen large.

If you're really concerned about loss of capital, you can go with a relatively moderate allocation like 50 percent stocks (or, more likely, stock funds) and 50 percent bonds (actually, I think bond funds would work better, and funds with short- to intermediate maturities).

If you think even that is too wild a ride, then scale back the stock-fund portion of your portfolio and put more into bonds and/or money funds.

In fact, you can make things even simpler by investing in a "target" retirement fund.

You pick a fund with a target date that roughly matches the year you intend to retire -- say, 2020 -- and you get a portfolio that already contains a mix of stocks, bonds and cash that's appropriate for someone retiring at that time.

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What's more, the fund automatically shifts money from stocks to bonds and cash as you age, in effect providing more principal protection as you near retirement. All in all, I think they're a good deal for people who don't want to create a portfolio on their own and don't want to bother with changing their stocks-bonds mix over time. For more on how these funds work, click here.

So that is what I suggest. Set your sights a little higher, but do it in a way that hedges the risk. When retirement rolls around and you're depending on your IRA rollover and other assets for spending cash, I think you'll be glad you took this route.

Walter Updegrave is a senior editor at MONEY Magazine and is the author of "We're Not in Kansas Anymore: Strategies for Retiring Rich in a Totally Changed World."