It's always fascinating to read stories about average, everyday people who built fortunes by regularly investing small amounts over long periods of time in companies such as General Electric (NYSE:GE), ConocoPhillips (NYSE:COP), or Wal-Mart (NYSE:WMT).

If you worked for these companies, and/or regularly "trickled" money into them over the years, this is quite feasible -- GE, ConocoPhillips and Wal-Mart have returned 8%, 10%, and 14% annually over the past two decades, respectively, even after taking into account the pummeling that GE has experienced in the past year.

But you can also get market-beating returns by buying into great companies at more opportune times -- whenever the stock goes on sale. Rather than regularly investing small, fixed amounts, investors can use the simple method of buying a stock in portions to manage risk and boost returns. And now would definitely count as one of those opportune times to buy cheap stocks.

First, find a solid business
Of course, every situation is different, but big returns on investments always come on the backs of fundamentally strong businesses. And if you're confident that you've purchased shares in a great company, why wouldn't you consider buying again, particularly if the stock price is significantly below intrinsic value? Especially in pessimistic markets (like today's), fundamentally strong businesses can be bought for good prices -- or even downright outrageously cheap.

For large, stable companies, buying more shares when the outlook for them is bleak can be rewarding. For instance, buying more Altria (NYSE:MO) back at the peak of investors' pessimism over tobacco lawsuits would have juiced your returns considerably -- the stock is up more than 530% from its low in 2000.

For younger, riskier companies, a strategy of acquiring shares in portions is a smart play. It limits your initial outlay and reduces your exposure to significant drops should the company falter or broader economic conditions change.

For example, look at Motley Fool Stock Advisor recommendation Mobile Mini (NASDAQ:MINI), a company that leases portable storage units for commercial and residential markets. From 1997 to the beginning of 2002, Mobile Mini's stock soared nearly tenfold as the company capitalized on rising demand for storage. Then, in an abrupt six-month period afterward, the stock shed roughly 70% of its value.

When demand for portable units dropped with the slowing economy, margins began to shrink, and investors poured out of Mobile Mini stock. But the fundamental business operations remained intact. Money invested when the outlook was bleakest is still up more than 130% -- even with shares well off their high three years ago. The larger economic conditions had only a temporary impact on Mobile Mini's solid business model before, and recent turmoil once again gives investors a cheaper price to consider.

Buy again                                                                             
Other companies, such as NVIDIA (NASDAQ:NVDA) and Research In Motion (NASDAQ:RIMM), have experienced big drops in share price at some point, only to come roaring back. Investors who focused on the underlying businesses, rather than the stock prices, were more likely to turn the event into an opportunity.

The final caveat with this method is to ensure that you aren't throwing good money at a truly deteriorating company -- hence the importance of understanding the underlying business. In their Motley Fool Stock Advisor service, David and Tom Gardner track all of their investments and re-recommend promising companies when the price is right.

If you'd like to see which stocks they recommend you buy again -- and again and again -- you can click here and get a 30-day trial of the service for free.

This article was originally published Feb. 12, 2007. It has been updated.

Fool contributor Dave Mock buys pogs again and again -- more for sentimental than intrinsic value. He owns shares of NVIDIA and Mobile Mini. Wal-Mart Stores is a Motley Fool Inside Value pick. Mobile Mini and NVIDIA are Stock Advisor recommendations. The Motley Fool's disclosure policy keeps a shopping list handy.