NEW YORK (CNN/Money) - I'm 29 years old and have 30 percent of my Roth IRA contributions in cash because I've been uncertain about stock mutual funds over the last two years. I am now planning to move that cash to stock funds along with my new contribution. Any recommendations on funds?

-- Per Peterson, Washington, DC

Before I get to my fund recommendations, I have a question: What is a nice young person like you doing fooling around with market timing? Do you really think that you can tell in advance when stocks are going to climb or fall and that you can move your money in and out so you ride the ups and avoid the downs?

Oh, at first glance your decision to hold a big chunk of your Roth IRA in cash instead of stocks seems like a wise one. After all, over the past two years, the Standard & Poor's 500 index -- a decent proxy for the broad stock market -- is down about 2 percent. Throw in a small dividend payment and we're basically talking a flat to very slightly positive return.

A money-market fund, by contrast, would have returned maybe 1 percent a year or so during that time, giving you a positive return of roughly 2 percent. So your cash stash did slightly better than stocks over the past two years.

But does this indicate some sort of clairvoyance on your part? I don't think so. Because if you really had any idea of the short-term future path of stock prices, you wouldn't have sat in cash the entire past two years. You would have moved out of your cash stash and into stocks in early October, 2002 when stock prices bottomed out.

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Had you done that, you would have ridden the stock rebound of the past year and now be sitting on a return of about 40 percent with the portion of your portfolio that instead languished in a low-yielding cash account.

The fact of the matter, of course, is that it's extremely difficult to shift money in and out of stocks to profit from the ups and downs of the stock market. Indeed, I'd say virtually impossible, and not worth trying.

Which is why I recommend instead that investors like yourself pick a mix of stocks and bonds (and perhaps some cash) that's appropriate given your investing time horizon and tolerance for risk, and then stick to that mix of assets regardless of what's going on in the market.

This is especially true for money one is investing for retirement, which I assume is the case with your Roth IRA money.

Now, people can argue back and forth as to exactly how much of one's retirement savings portfolio should go into stocks vs. bonds vs. cash. In my new book,"We're Not In Kansas Anymore: Strategies for Retiring Rich In a Totally Changed World", I devote an entire chapter to retirement investing strategies and offer model portfolio allocations for different types of investors.

Given your young age, I'd say that you should definitely be tilting your mix toward stocks (or stock funds, as the case may be) and keeping a relatively small portion of your Roth money -- say, 10 to 20 percent -- in bonds. Frankly, I don't see a need for cash, unless you have no other cash or savings reserve.

Whatever mix you set, leave it alone, except to rebalance once a year to bring your portfolio back to its original proportions.

As for fund recommendations, I like to keep things simple. You could put the stock portion of your holdings in a stock index fund that tracks a broad market index such as the S&P 500 or the Wilshire 5000 and then throw the bond money into a bond index fund that tracks the Lehman Bros. Aggregate Bond Index.

As Emeril would say, BAM! Just like that you've got a fully diversified portfolio of stocks and bonds.

If you want to keep it even more simple, you can check out a life-cycle fund. These funds can work in a variety of ways, but the general idea is that the fund owns both stocks and bonds and tilts the mix from stocks to bonds as you age. For more on investing via my index-fund or life-cycle approach, click here.

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if you want to get fancy, you can round out your portfolio with actively managed funds (which you can choose with the help of our Fund Screener. But I don't think you'd be giving up much in the way of returns if you stick to the simple approach. In fact, I think you're more likely to do better.

So forget this fantasy of darting back and forth between cash and stocks. Instead, just set your portfolio mix, invest in a few funds and then turn your attention to something important -- like enjoying life.

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." He also answers viewers' questions on CNNfn's Money & Markets at 4:40 PM on Mondays.