I'm 25 and off to a good start saving for retirement. But I'm scared stiff about how to invest my Roth IRA because a poor decision could cost me hundreds of thousands of dollars by the time I retire. Any advice? Andy C., Des Moines, Iowa

Relax, there's no need to work yourself into a lather. Even though many pros like to make investing seem complicated -- probably so you'll hire them to manage your money -- it's really not all that difficult.

Sure, you'll make some mistakes. We all do. But as long as you follow a few key principles like keeping it simple, holding the line on costs, diversifying broadly and ignoring the jabber of pundits who advocate constant buying and selling, any flubs you make aren't likely to wreak mortal damage.

Take solace in the fact that you're already doing the single most important thing to assure a secure future: You're actually saving. Wall Street types may cringe when I say this, but contributing to a retirement account regularly throughout your career is more important than investing prowess for building wealth over the long term.

That said, since you're going to the trouble to put bucks aside in your Roth IRA, you might as well earn a decent return. Here are three ways you can do that, ranging from easy to easier to easiest, without getting obsessive-compulsive about it.

The easy way. Your aim as a long-term investor should be to build a diversified portfolio of stocks and bonds and then stick with it, except to rebalance every year or so.

You can create that stocks-bonds mix simply and effectively by investing in just three funds: a total stock market index fund, a total bond market index fund and a total international stock index fund, each of which you can find on our MONEY 70 list.

This three-fund combo will give you exposure to the entire U.S. stock market, virtually all publicly-traded foreign stocks and the full gamut of taxable investment-grade domestic bonds.

In short, you'll put together the building blocks for a well-balanced portfolio -- and you'll do it on the cheap. Your annual costs should come in below 0.25% a year, or less than a quarter of what the typical mutual fund charges.

As for how to divvy up your money among these three components, there's no "official" blend. But considering that you're young and have plenty of time to ride out market setbacks, you'll want to lean heavily toward stocks, which have the potential to generate the highest long-term gains.

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Everyone makes some retirement investing mistakes