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 on to 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 Nortel Networks' (NYSE:NT) performance over the past couple of years.

We can establish that 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 mean a big difference to investors.

Over the past five years, Gap (NYSE:GPS) grew its total revenue at 2% 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, each grew their revenues at 19% per annum over the same period. As you might expect, investors did much better in Jos. A. Bank and Abercrombie. $1,000 investments in each grew into $8,760 and $3,037, respectively, while Gap returned just $1,396. 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. Apple Computer's (NASDAQ:AAPL) iPod, for instance, is by far the most popular mp3 player in the world, leaving its closest competitors like Microsoft and SanDisk (NASDAQ:SNDK) to duke it out for a No. 2 product. Apple achieved its success largely by exclusively linking its superior iPod product to its easy-to-use music downloading service, iTunes. Before any other competitor could implement a similar strategy, Apple had already made its name synonymous with portable digital music. This competitive advantage has helped Apple shares return 30% per year to shareholders over the past 10 years.

Similarly, Paychex has enjoyed incredible success by constantly improving its sustainable competitive advantages in payroll services and human resources for small businesses. It currently serves about 543,000 clients and is constantly growing. Although its payroll services are perhaps the most well-known, its human resources services business is in a relatively young industry and could be the segment that drives the company's growth in the next decade. Shareholders have ridden its competitive advantage to handsome profits. If you had invested $1,000 in Paychex in 1990, you would now have about $74,000. Now that's been great sustainable growth.

3. A good price
One of the biggest mistakes 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 more common for investors to pay so much that it's almost impossible to make a decent profit, even if the growth continues.

Take Business Objects, for example. Its software products are still recognized as leading the business intelligence market, and the company has continued to grow its revenue at a good rate. Yet the stock is well off its highs, mainly because it was so overpriced six years ago that no amount of performance would have kept propelling it up.

So 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, since 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 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.

Fool's final word
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 these companies are worth significantly more than weaker companies. At the same time, value investors also know that if you overpay for that growth, then 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 does not have a position in any security discussed in this article. Gap is an Inside Value and Stock Advisor pick. Microsoft is an Inside Value selection. The Fool has adisclosure policy.