Most loyal Fool readers know how we feel about selling. If you've found a great company with top-notch management and a strong competitive advantage, the best time to sell is almost never.

But that doesn't mean we hold on blindly. Things change, even with the greatest of companies. That's why we're constantly evaluating our stocks and watching for the danger signs that can torpedo our portfolios.

Today, I'd like to share three rules for selling, as set forth by Fool co-founder Tom Gardner for his Motley Fool Stock Advisor members.

1. Selfish management
Tom calls this the "worst possible development" for any of his companies. If the executive team starts worrying more about lining its own pockets than about creating value for the business, it's time to let go. For clues, keep an eye on excessive compensation, aggressive accounting, active insider selling, and declining market share. A classic example occurred at Fannie Mae (NYSE:FNM) a few years ago under a previous management team.

2. Competitive disadvantages
Competitive advantages are what lead businesses to high returns on capital and equity. These advantages could be a result of many things -- for instance, Disney (NYSE:DIS) and Nike (NYSE:NKE), though in different industries, use their strong brand names to attract more sales and attain greater pricing power. But if your company is facing weak pricing power, a declining customer base, and lower market share, it's likely operating at a competitive disadvantage.

3. An unstable financial model
Let's consider stable models first. Think of Logitech (NASDAQ:LOGI) and current Stock Advisor recommendation Bed Bath & Beyond (NASDAQ:BBBY). They're known for stable or rising margins, tight control over working capital, steadily increasing sales, loads of cash from operations, and a huge surplus on the balance sheet. Conversely, companies heading in the opposite direction in two or more of these categories are showing a big red flag.

What about valuation?
Obviously, a stock carrying a sky-high valuation is a candidate for selling. But this is the toughest call of all. If properly valuing a company were so easy, after all, everyone would be rich ... happily buying low and selling high. In One Up on Wall Street, Peter Lynch recounts agonizing over when to sell as Warner Communications (now a subsidiary of Time Warner) climbed from $26 to $32 in 1977. With 3% of his Fidelity Magellan fund invested in Warner, he pulled the trigger at $38 -- and agonized a lot more as the stock reached a near-term high of $180! So tread carefully here; it takes a large number of accurate valuation-based sell calls to make up for just one missed multibagger.

But the three sell signs above aren't too hard to spot. Tom and his brother, David, have employed accurate selling and buying guidelines on their way to outstanding performance in Stock Advisor -- 75% total average returns versus 29% for equal amounts invested in the S&P 500. You can get a look at the service, including their top five stocks for new money now, free of charge with a 30-day trial.

This article was originally published April 12, 2006. It has been updated.

Rex Moore is brought to you by the letter "R." He does not own shares of any companies mentioned in this story. The Motley Fool owns shares of Bed Bath & Beyond. Time Warner, Disney, and Bed Bath & Beyond are all Stock Advisor recommendations. Bed Bath & Beyond is also an Inside Value selection. Fannie Mae is a former Inside Value recommendation. The Fool has a disclosure policy.