As an investor, you should always be looking for growth. Share prices tend to follow a company's value, so investors should seek firms that are increasingly 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 at Nortel Networks' (NYSE:NT) performance over the past couple of 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
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, Gap (NYSE:GPS) has grown its revenues at around 3% per year. Not exactly the kind of growth you write home about. Jos. A. Bank (NASDAQ:JOSB) and Abercrombie & Fitch (NYSE:ANF), on the other hand, grew their revenues at 20% and 19% per annum over the same period. As you might expect, investors did much better in Jos. A. Bank and Abercrombie. A $1,000 investment in each would be worth $2,969 and $4,041, respectively, while Gap shares would be worth just $1,408 today. 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.

So, you should pay as much attention to the competitive position of the business as you do to the rate of growth. Wal-Mart (NYSE:WMT) has enjoyed incredible success by focusing on one important sustainable competitive advantage -- selling at the lowest possible prices. And shareholders have ridden that advantage to handsome profits. If you invested $1,000 in Wal-Mart in 1980, you would now have some $300,000. Now that's 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.

Take for example. The company has been growing revenue at a blistering pace from the moment it came public. Yet the stock is just now revisiting its 1999 highs, mainly because it was so overpriced eight years ago that no amount of performance would have kept propelling it upward. In fact, investors in late 1999 would have been better served by buying shares in Barnes & Noble (NYSE:BKS), even though top-line growth at the bricks-and-mortar bookseller has been stagnant for years.

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 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 be on the lookout 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 does not own shares of any company mentioned in this article. is a Stock Advisor selection. Wal-Mart is an Inside Value pick. Gap is both an Inside Value and Stock Advisor recommendation. The Fool has a disclosure policy.