From time to time, you'll read reports of one corporate insider or another selling shares. This is often followed by an interpretation that the sales signal trouble for the company. Unfortunately, insider sales are often motivated by other factors, like the need for liquidity (maybe Junior crashed the Porsche) or a desire to diversify holdings (insiders get a lot of stock in compensation packages, which ends up in their having an overly concentrated portfolio).
Insider purchases, on the other hand, are usually a much more reliable signal. The Motley Fool has a good piece on insider selling that expands on these ideas. Click here to read the whole thing.
If you're interested in learning more about using insider trading information to make investment decisions, Nejat Seyhun's book on insider trading Investment Intelligence from Insider Trading is probably the best treatment of the topic I've read. He's got serious academic credentials on the tropic (he's done more path-breaking work in insider trading than anyone else). It's not only extremely thorough, but also a pretty easy read (it's written for the layman).