As an investor, you should always be on the lookout for growth. Share prices tend to follow a company's value, so investors should seek companies that are rising in value. Truly phenomenal stock market returns are made by holding superior companies that grow relentlessly for decades.

But to actually identify the best growth stocks, you can't just look for companies with the highest projected growth rates. After all, if the market starts to lose faith in the company's prospects, the fall can be horrendous. Just look what happened to Netflix (Nasdaq: NFLX) at the end of April.

The best growth stocks offer both huge upside potential and a margin of safety. As such, they should satisfy three conditions:

1. A good growth rate
All else being equal, fast growth is better than slow growth. But because of compounding, even relatively small changes in the growth rate can make a big difference to investors.

Over the past five years, American Eagle (NYSE: AEO) has grown its net income at 35% per year. Meanwhile, Nordstrom (NYSE: JWN) has grown its net income at an incredible 51% annual clip over that same time. As you might expect, investors in Nordstrom did much better. A $1,000 investment in Nordstrom would be worth $4,237 today, while the same investment in American Eagle shares would be worth "just" $3,421. It can pay to find the fastest-growing stock in the industry.

2. Sustainability
But to achieve truly great results, you need to look beyond growth estimates. One of the biggest blind spots for most growth investors is focusing on the growth rate, but ignoring the sustainability of that growth.

This myopia was one of the main causes of the tech bubble. People started paying high prices for third-rate companies sporting high growth projections but few competitive advantages. Such investors were hurt badly when the bubble popped and the market for the companies they invested in disappeared.

You should pay as much attention to a business's competitive position as to its rate of growth. CME Group (NYSE: CME), which runs the Chicago Mercantile Exchange, one of the main U.S. futures and options exchanges, has network effects that kept competitors away. Both buyers and sellers flock to the busiest exchange, because the more traders at an exchange, the better liquidity, the tighter the spreads, and the lower the transaction costs. This competitive advantage has helped the Chicago Mercantile Exchange grow for more than 100 years.

3. A good price
One of the biggest mistakes that investors make is paying too much for growth. Occasionally, you can pay a steep price and strong sustained growth will bail you out. But it's common for investors to pay so much that it's almost impossible to make a decent profit, even if the growth continues.

The tremendous potential of (Nasdaq: AMZN) and Yahoo! (Nasdaq: YHOO) was obvious to everyone in 1999. These are two of the most successful Internet businesses around, but their stocks have hardly been rewarding to investors who bought during the tech bubble. Amazon has produced modest gains for anyone who bought eight years ago, while Yahoo! is still underwater.

So before buying a growth stock, make sure it's undervalued, or at least fairly priced. A discounted cash flow (DCF) calculation is a great way to work out the fair value of a growth company. If you don't know how, our Motley Fool Inside Value newsletter has an easy-to-use DCF calculator for subscribers. (A free trial is available.) With a few quick clicks, it can tell you what you're paying for, and help you avoid paying too much.

The Foolish bottom line
These three ideas are central to a value investment strategy. Value investors aren't just looking for unpopular stocks. If anything, like Warren Buffett, we prefer to purchase strong companies with excellent growth prospects, because we recognize that these companies are worth significantly more than weaker companies. At the same time, value investors also know that if you overpay for that growth, you're increasing your risk while reducing potential profits.

The best growth stocks offer sustainable growth at a reasonable price. When you find this sort of stock, the long-term profits can be immense. It pays to constantly search for such businesses, the kind that make up the core of our Inside Value portfolio. If you're looking for investment ideas, you can check it out with a 30-day free guest pass.

This article was originally published on July 14, 2006. It has been updated.

Fool contributor Richard Gibbons was hoping for sustainable growth, but he stopped at 6'2". Richard owns shares of American Eagle. American Eagle, Amazon, and Netflix are Stock Advisor selections. The Motley Fool owns shares of American Eagle. The Fool has a disclosure policy.