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 Blockbuster's (NYSE:BBI) performance over the past couple of years.

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, Honda Motor (NYSE:HMC) grew its total revenue at nearly 9% per year -- not too shabby. However, rival Toyota Motor (NYSE:TM) grew its revenue at 11% per year over the same period. As you might expect, investors did much better in Toyota. A $1,000 investment in Toyota would have grown into $2,648, while an equivalent amount invested in Honda would be worth only $1,867. 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 Sony (NYSE:SNE) and SanDisk (NASDAQ:SNDK) to duke it out for No. 2. Apple achieved its success largely by exclusively linking its superior iPod product to its easy-to-use music downloading service, iTunes. Before any 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 nearly 1,500% since the iPod was introduced in October 2001.

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 Advanced Micro Devices, for example. Its processors are still employed in scores of PCs and other electronic devices, 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 seven years ago that no amount of performance could 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 (DCF) calculation. If you don't know how, the 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 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". Richard does not own any of the securities mentioned in this article. The Fool has a disclosure policy.