As an investor, you should always be looking for growth. Share prices tend to follow a company's value, so investors should seek companies that are increasingly more valuable -- growth companies. The 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 have to take a step beyond looking for the 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 at Taser's (NASDAQ:TASR) performance over the past couple years.

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
It might seem a bit obvious to anyone who has passed toilet training, but the most successful growth stocks are the ones that grow. And all else being equal, fast growth is better than slow growth. Because of compounding, even relatively small changes in a growth rate can mean a big difference to investors.

In the past 10 years, Nordstrom (NYSE:JWN) has grown its revenue 6.5% per annum. American Eagle Outfitters (NASDAQ:AEOS), on the other hand, grew its revenue per share at an impressive 22.3% rate. As you might expect, investors did much better in American Eagle. A $1,000 investment in American Eagle grew to almost $75,000, while Nordstrom returned $6,000. 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 and ignoring the sustainability of the 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.

So you should pay as much attention to the competitive position of the business as you do to the rate of growth. CBOT Holdings (NYSE:BOT), which runs the main U.S. futures and options exchange, has networking effects that keeps 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 Board of Trade grow for more than 150 years.

Similarly, Dell has enjoyed incredible success by focusing on one important sustainable competitive advantage: building the most efficient logistics system in the business, allowing it to sell PCs at a lower price than its major competitors. And shareholders have ridden that advantage to handsome profits. If you invested $1,000 in Dell in August 1990, you would now have more than $200,000. Now, that's been great sustainable growth.

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.

Cisco Systems (NASDAQ:CSCO) is the dominant name in networking, but before the bubble popped, it was trading at completely unreasonable multiples for such a big business. Consequently, the stock is still 68% off its March 2000 highs.

Before buying a growth stock, make sure it's undervalued, or at least fairly priced. A great way to work out the fair value of a growth company is by using a discounted cash flow calculation, because these formulas will take the company's growth into account. If you don't know how to do these calculations, the Motley Fool Inside Value newsletter has an easy-to-use discounted cash flow calculator here for subscribers (a free trial is also 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 Buffett, we prefer to purchase strong companies with excellent growth prospects because we recognize that such 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 both increasing risk and 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, so it pays to constantly be on the lookout for these businesses. Such companies 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". He owns calls in Cisco but does not have a position in any other security discussed in this article. Dell is an Inside Value and Stock Advisor recommendation. Taser is a Rule Breakers pick. American Eagle is also a Stock Advisor pick. The Fool has a disclosure policy.