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.

So, 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, Nike (NYSE:NKE) has grown its revenue by 10.5% annually. Steve Madden (NASDAQ:SHOO), on the other hand, grew its revenue at an impressive 14.3% rate. As you might expect, investors did much better in Steve Madden's shoes. A $5,000 investment in Steve Madden has grown to $13,100, while Nike returned $10,450. 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. EBay (NASDAQ:EBAY) prospered throughout the tech bust because it had networking effects that kept competitors away. Sellers wanted to list on the website with the most potential buyers, and the buyers wanted to buy items on the site that offered the widest selection. Thus, once eBay had established itself, it was very difficult for anyone else to compete. Similarly, 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 more than $400,000. 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.

Take Adobe Systems (NASDAQ:ADBE). Its software products are still top of their categories, and the company had similar revenue growth rates before and after the tech bubble. Yet the stock is just now revisiting its 2000 highs, mainly because it was so overpriced seven years ago that no amount of performance would have kept propelling it up. EMC (NYSE:EMC) is in a similar position. It's a dominant name in information storage, but before the bubble popped, it was trading at completely unreasonable multiples for such a big business. Consequently, the stock has been down for years.

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, 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 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 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, 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 owns shares of Steve Madden. EBay is a Stock Advisor recommendation. Wal-Mart is an Inside Value pick. The Fool has a disclosure policy.