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

If you worked for these companies, and/or regularly "trickled" money in them over the years, this is quite feasible -- Citigroup, GE, and Wal-Mart have returned 14.2%, 15.9%, and 26.6% annually over the past three 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 simple methods 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 back of fundamentally strong businesses. And if you're confident you've already purchased shares in a great company, why wouldn't you at least 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 350% 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 Mobile Mini (NASDAQ:MINI), a company that leases portable storage units for commercial and residential markets. From 1997 to 2002, Mobile Mini's stock soared nearly 10-fold as the company capitalized on rising demand for storage. Then, in an abrupt six-month period afterward, the stock shed 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. Investors who simply held have still managed to match the broader market return. But money invested when the outlook was bleak is now up more than 400%. The larger economic conditions had only a temporary impact on Mobile Mini's solid, proven business model.

Buy again
Other companies such as NVIDIA (NASDAQ:NVDA) and Bio-Rad Laboratories (NYSE:BIO) have had big drops in share price in the past few years, only to come roaring back afterward. Investors who focused on the underlying business, rather than the stock prices, were more likely to grab the opportunity for a significant profit.

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 their investments for re-recommendations to buy, and this diligence pays off. The average performance of companies they re-recommend for investments is 80.5%, compared to 67.1% performance of all company recommendations. 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.

Fool contributor Dave Mock buys pogs again and again -- more for sentimental than intrinsic value. He owns shares of NVIDIA and Mobile Mini. NVIDIA is a Motley Fool Stock Advisor recommendation. Wal-Mart is a Motley Fool Inside Value recommendation. A longtime Fool, Dave is also the author ofThe Qualcomm Equation. The Motley Fool has a disclosure policy.