Psst! Wanna know the secret to finding the next home run stock?

You may already own it.

Yep, that's right -- it may already be in your portfolio.

You've already chosen some of your best investing ideas. You've done the winnowing and narrowing. So why wouldn't you double down on those top picks?

Warren Buffett was fond of saying that people should invest as though they had a lifetime investment punch card with only 20 punches available -- and only the best ideas would be worth spending those punches. Chances are, your portfolio's punch card already has one or two or three companies on it that represent some of your best thinking.

As Peter Lynch said in his best-seller Beating the Street, "The best stock to buy may be the one you already own."

The secret to market-beating returns
It's hard enough to find one stock that's going to double in price -- but bailing out after it hits some magical preset percentage means you think you can do it all over again. It's simpler to hold fast to your winners and let them keep winning.

Here's a list of stocks that have doubled or more over the past year. Great, right? But look at their returns over the past two years. Investors in these stocks would have done well to reinvest -- without reinventing the wheel.

Stock

1-Yr Return

2-Yr Return

Cleveland-Cliffs (NYSE:CLF)

185%

454%

PotashCorp (NYSE:POT)

120%

462%

Cal-Maine Foods (NASDAQ:CALM)

130%

495%

Mosaic (NYSE:MOS)

181%

577%

Sequenom (NASDAQ:SQNM)

348%

845%

The magic of re-upping
Reinvesting in a portfolio's winners is a formula for success that has served buy-and-hold investors for years. It works hand in hand with what Fool co-found David Gardner considers the greatest investing secret of all. "Finding good companies and holding those positions tenaciously over time can yield multiples upon multiples of your original investment," David wrote. "That's what great investors do."

David has used the recipe himself at his Motley Fool Stock Advisor investment service, recommending both comic-book-hero stable Marvel Entertainment (NYSE:MVL) and online movie-rental outfit Netflix (NFLX) multiple times.

With just two stocks and eight recommendations, David is whomping the market. And that's what investors should remember: The quality of the stocks you're selecting is more important than the quantity.

And just because a stock has done well doesn't mean it can't continue to do well by its investors. Wal-Mart has returned 21% annually since its IPO. At just about any time over those 23 years, you could have bought shares -- and bought them again! -- and realized fabulous returns.

What are tomorrow's monster stocks?
While not every investment will be the next Wal-Mart -- indeed, few companies can do that well -- you needn't continuously go out and find new investment ideas to achieve market-beating returns. Your old ideas may still be your best.

Instead of looking for the next opportunity, David recommends that you hold your great businesses doggedly, and consider buying even more as the stock price drops. If you'd like to see what he recommends you buy right now, click here to join Stock Advisor free for 30 days -- there's no obligation to subscribe. The stocks he suggests may very well join the ranks of those you invest in again and again.

Fool contributor Rich Duprey owns shares of Wal-Mart, but he does not have a financial position in any of the other stocks mentioned in this article. Wal-Mart is a Motley Fool Inside Value selection. Netflix and Marvel Entertainment are Stock Advisorrecommendations. The Motley Fool has a disclosure policy.