Investors often fall in love with the companies they own, which can be a mistake: If a situation changes, you need to change your opinion of the stock. But how do you actually know when the situation has changed? One way is to buy the products and services sold by the companies you've invested in.

Here's why it's a good idea, even though some might scorn the practice.

Peter Lynch, who famously managed Fidelity Magellan Fund for years, wrote a book called One Up on Wall Street that suggests, basically, to buy what you know. So if you were to go to a mall and see that a store was constantly crowded with customers, it could be worth a look to see if it was public.

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Source Fool.com