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

If you worked for these companies, and/or regularly "trickled" money into them over the years, this is quite feasible -- Citigroup, GE, and Wal-Mart have returned 15.8%, 14.8%, and 15.3% annually over the past two decades, respectively.

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 in small, fixed amounts, investors can use the simple method of buying a stock in portions to manage risk and boost returns.

First, find a solid business
Of course, every situation is different, but great 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? If the business and its model are still fundamentally sound, it's a golden opportunity.

For larger, more stable companies, simply buying more shares when the outlook 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 650% 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 gives you a chance to buy again if shares experience an unwarranted drop.

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 now up nearly 200% -- even after the stock has shed 50% of its value in the last six months with the return of another poor economic outlook. 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 Boeing (NYSE: BA) have similarly experienced big drops in share price, only to come roaring back afterward. Investors who focused on the underlying businesses, rather than the stock prices, were more likely to grab the opportunity for significant profits.

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 rerecommend promising companies when the price is right.

If you'd like to see which stocks that 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 Mobile Mini and NVIDIA, which both happen to be Stock Advisor recommendations. A longtime Fool, Dave is also the author of The Qualcomm Equation. The Motley Fool's disclosure policy keeps a shopping list handy.