For years, Warren Buffett has focused on finding the strongest companies with the best competitive positions, and buying them when they're cheap. That strategy has made him billions, and it's perfectly suited to today's market.

After all, these days, you can buy many of the best companies in the world for less than their fair value. You just need to be brave enough to face the volatility.

But ironically, the biggest risk to this strategy isn't the daily volatility. If a company's truly strong, it will be able to survive even if we hit a depression. The real risk lies in buying a company whose competitive position is weakening, or weaker than it appears.

Deceptively weak barriers
Often, a business seems to have strong barriers against competition simply because it has a well-known brand and large market share. In markets where there are significant economies of scale, such as many manufacturing or distribution businesses, market share can be a huge barrier. But if the economies of scale are relatively insignificant, a competitive advantage because of market share can be far weaker than it appears.

Take Charles Schwab. It has a good brand name and billions in assets under management, but its revenue last year was below its 2000 level. A major problem is that there are few barriers to creating an online brokerage. Interactive Brokers and thinkorswim have both made big splashes in the past few years by offering superior trading technology, while E*TRADE has also grown its brokerage top line significantly. Meanwhile, retail banks such as Wells Fargo (NYSE: WFC) and a partnership between Morgan Stanley and Citigroup's (NYSE: C) Smith Barney have begun offering online brokerage services.

If Schwab truly had a huge moat, these competitors would have had a difficult time gaining any traction, which they have. Thus far, thanks to the brand and stickiness of assets under management, Schwab has actually been able to grow its margins, but it's unclear how long that will last, given increasing competition from depository banks that can leverage existing clients into brokerage services.

Times change
The impact of fewer people reading newspapers has been obvious for years, as advertising dollars have fled from papers to the Internet. But societal changes are affecting television networks as well. CBS has had declining revenue for years, and if you exclude the effects of the Olympics and acquisitions, General Electric's NBC unit has also had limited growth. The world is changing, and it's hurting both these networks.

While TV viewing is at all-time highs, couch potatoes have more channels than ever before, meaning the market is more fragmented. The rise of Internet television only increases market fragmentation. As if that weren't enough, personal video recorders (PVRs), such as those offered by TiVo (Nasdaq: TIVO), and-file sharing networks have made it much easier for consumers to skip commercials. It doesn't particularly matter to the value of their weakening moats that TiVo is expected to continue losing money for a few years; these changes will result in lower ad rates and weaken the competitive position of TV networks.

Watch out for technology
It's no coincidence that new technology is playing a big role in weakening the TV networks. Game-changing technology is one of the biggest risks that a company can face, and that risk isn't limited to high-tech businesses. Even a low-tech business like movie rentals can be affected.

For years, Blockbuster (NYSE: BBI) was the go-to movie rental chain. However, the advent of the Internet allowed Netflix (Nasdaq: NFLX) to offer movie rentals by mail or download. While Blockbuster's sales are in sharp decline, Netflix's are booming. Blockbuster recognizes the risk and isn't just sitting back. It now offers video downloads. But, it's unlikely that the company will achieve the same dominance as it enters a new arena to take on Netflix's first-mover position. It trails other runners-up like Wal-Mart (NYSE: WMT) and Best Buy (NYSE: BBY) in online video delivery -- both of which are determined to sell videos online and have very deep pockets.

So when looking at beaten-down stocks, be particularly aware of technological threats to the business.

The Foolish bottom line
That said, this doesn't mean that you should never buy any company whose moat has weakened. Even from an eroding competitive position, some blue chips can generate cash for decades. But make sure that the price you pay is cheap even considering the impoverished prospects of the business.

If you are looking to take advantage of the market decline, our Inside Value team spends a lot of time thinking about moats, and we've identified many excellent stocks that look exceptionally cheap today. You can read about them for 30 days for free by clicking here.

This article was originally published May 22, 2009. It has been updated.

Fool contributor Richard Gibbons is looking to buy a swamp monster to put in his moat. He doesn't own shares of any companies mentioned. Best Buy and Charles Schwab are Stock Advisor recommendations. Best Buy, Netflix, and Wal-Mart are Inside Value selections. The Fool owns shares of Best Buy. The Fool's disclosure policy wants to be an Alt-A mortgage when it grows up.