We all invest for the same reason: to make money. And to make money investing, we need to know two key things: when to buy and when to sell.

If you can buy something for $1 and turn around and sell it for $2, then you've made money. If, on the other hand, you buy for a buck and can't find someone willing to buy it for more than $0.50, you've lost money. Clearly, to make money at investing, the goal is to buy low and sell high. More than half a century ago, Benjamin Graham, the pioneer of value investing, came up with a simple way to do just that -- a concept known as the "margin of safety." By deploying this technique, investors greatly increase the chance that they won't lose their hats and the likelihood that they'll trounce other investors.

Following in Graham's footsteps, Bill Miller, who runs Legg Mason Value Trust, has beaten the market for 14 consecutive years -- practically unheard of in the mutual fund industry. Miller's long-run performance pales in comparison with that of Warren Buffett, a former pupil of Graham's and current head of Berkshire Hathaway. What's more, Graham's margin of safety is something we put to good use here at the Motley Fool Inside Value newsletter service.

Know a company's true worth
The key to success is a clear understanding of a company's true worth. With that knowledge in hand, buying low and selling high becomes a simple matter of waiting, buying a stock only when it falls below the company's true worth by a tempting margin. Once you own it, you need to keep tracking the company's value. When the stock rises to an uncomfortably high premium to its true worth, sell it. The central lesson: All buying and selling decisions should be guided by comparing a company's stock price with its true worth, not by some vague notion of what the hot stock of the moment is.

My friend and colleague Philip Durell follows that philosophy as advisor/analyst at Inside Value. His selections as a whole have handily beaten the market's return since the newsletter's inception last year. His record is added proof that using a margin of safety truly does work.

Philip has beaten the market without help from the likes of commodity-driven businesses like coal-mining company Arch Coal (NYSE:ACI), whose shares may have blown the market away over the past year but which owes its run more to the hike in the price of energy than to any particular improvements in its operations. Instead, Philip has relied on companies with competitive moats, such as management consulting giant Accenture (NYSE:ACN). Because of its large size, strong reputation, and solid track record, Accenture can command premium prices for its services. Additionally, since many other consulting firms are attached to technology giants like IBM (NYSE:IBM) or SAP (NYSE:SAP), Accenture's independence in that area makes it easier to maintain the objectivity that helps it win customers.

Buying low with the margin of safety
Every company has what Graham calls an "intrinsic value," a measure of what it is really worth. While finding that value is part art and part analysis, there are simple steps you can take to get there. One of the most powerful tools in a value investor's toolkit is a discounted cash flow calculator, into which you put your estimate of how much cash the company will generate in the upcoming years. The calculator then tells you how much the company is worth today. Inside Value has just such a calculator available to subscribers. If you're already a subscriber, click here to access it. If not, click here to take a free 30-day trial to the newsletter and play with the calculator to your heart's content.

Once you've figured out what the company is worth, you can use that information to determine whether or not it has enough of Graham's margin of safety to be worth buying. Imagine buying fast-food titan McDonald's (NYSE:MCD) in March 2003. At the time, its shares traded below $12.50 a stub, thanks to the one-two punch of a struggling turnaround and fears of lawsuits essentially blaming the company for causing America's obesity epidemic. Now, thanks to an improvement in its operations and the legal environment, along with potential interest from real estate giant Vornado (NYSE:VNO) to help McDonald's unlock the value of its land, shares in the home of the Big Mac have rebounded to well above $30 a share -- more than doubling in less than two years, and returning to a much more rational level.

Selling high with the margin of safety
Logically, if a company trading below its intrinsic value is worth buying, then a company trading at or above its intrinsic value just might be a candidate for selling. For instance, in October 2004, I profiled struggling life insurance company Presidential Life (NASDAQ:PLFE), pegging my estimate of the company's fair value somewhere around $17.88. The rising dividends I had been hoping to see to justify that valuation have not materialized, yet the firm's shares have since risen well above $19 a stub. At this point, I am re-evaluating my stake in the business. If my estimates don't indicate that the company's value has increased enough to justify its higher price, I will happily take my capital gains and reinvest my money elsewhere.

The Foolish bottom line
Once you've figured out what a company is really worth, its margin of safety will tell you when it's time to buy and when it's time to sell. The lower the company's price with respect to that intrinsic value, the stronger the margin of safety, and the better the chance that buying that company will lead to a profitable investment. The higher the company's price with respect to that intrinsic value, the more that margin of safety has been reversed, and the better the chance that it's time to sell your position and take the extra profits from your bargain-hunting trip.

Like the idea of finally knowing how to buy low and sell high? Want more value-investing tips and techniques? Click here for a free 30-day trial to Inside Value, The Motley Fool's home of the margin of safety.Subscribefor a full year, and you'll also receive a free copy of Stocks 2006: The Investor's Guide to the Year Ahead, a $69 value.

This article was originally published on July 13, 2005. It has been updated.

At the time of publication, Fool contributor and Inside Value team memberChuck Salettaowned shares of Presidential Life. Accenture is an Inside Value pick. The Fool has adisclosure policy.