There's been a whole lot of pain in the market of late, forcing a lot of folks to sell stocks.

While there are always good reasons to sell, today I'd like to ponder three dumb reasons to sell.

1. I'm selling because the price is going down
Selling Morgan Stanley (NYSE: MS) or Bear Stearns (NYSE: BSC) because you think there will be more subprime pain to come -- pain not reflected in the current price -- is perfectly sensible. These companies created a mess for themselves, and their shareholders have been feeling the pain.

But selling a stock for no other reason than that the price is declining? That's plain dumb. If you hadn't factored into your valuation Bear's subprime exposure, then sure, sell it. Fast. But if you did, and are just fidgeting at the sight of red ink, you're forgetting that you're selling part of a living, breathing business -- not trading a slip of abstract paper.

2. I'm selling because someone told me the market is going lower
If you're selling off stocks because you read in a newspaper that a recession is now inevitable, or that the subprime crisis was only going to get worse, or that a multitude of "experts" predict that a bear market is nigh, then you're overlooking the trees for the forest.

See, as a retail investor, you have no control over the direction of the economy. Get used to it and deal with it. You do have control over the companies in your portfolio. If you've bought quality companies at reasonable prices, short-term economic movements should have little effect on the long-term returns of those businesses.

3. I'm selling and buying back in when the market bottoms
In the short term, no one knows which direction the stock market is going to move, or by how much. Trying to pick the bottom of the market is an utterly futile task. In a recent article, Foolish colleague Tim Hanson cited a study that illustrates exactly how futile market-timing is:

[IESE Business School professor Javier] Estrada studied 15 major global stock markets for periods ranging from 31 to 79 years, with the full data encompassing more than 160,000 trading days. What he found is "less than 0.1% of the days considered" actually matter to long-term returns, which means that "the odds against successful market timing are staggering."

Get used to this rough market
Selling in and of itself is not bad -- but selling for the three dumb reasons above will hurt the long-term results of your portfolio. Moreover, they'll lead you to a lot of the wrong investing behaviors: panic instead of patience, and a mind-set of trading stocks instead of owning businesses.

Remember: You don't sell the share price, you sell the company.

I'm not suggesting that investing is that easy, or that you should never sell. Shareholders of well-run businesses such as Best Buy (NYSE: BBY) and Staples (Nasdaq: SPLS) have seen their paper wealth decline, even though their companies haven't changed in any material way.

The way to calm yourself, though, is to follow what I call Warren Buffett's surefire investing strategy.

The surefire strategy
The Buffett maxim that I've seen quoted most amid market volatility is to "be fearful when others are greedy and greedy when others are fearful."

Buffett has done just that for decades: His investment in American Express (NYSE: AXP) after the "salad oil scandal" has brought him a fair degree of success, wouldn't you say?

So here's a thought: Instead of selling, consider taking advantage of the falling stock market to buy. Which leads me to Buffett's surefire investing strategy: Buy great companies when they are trading at cheap prices.

Buy for smart reasons, don't sell for dumb ones
But you need to be prepared. If you are prepared, you'll know exactly which shares to buy, and at what price to buy them. Keep your watch list close by, and do your best to value the companies on said list. (Here's a great article to help with that.)

Echoing Buffett, Motley Fool Inside Value advisor Philip Durell believes this market is a compelling one for long-term-minded investors. As he puts it, "The market's upheavals are a source of great opportunity for us."

I'll even give you an idea for your watch list: USG (NYSE: USG), a recommendation in Inside Value and a holding in Berkshire Hathaway's portfolio. USG manufactures and distributes wallboard. Our Inside Value team believes its shares to be trading at a discount to the company's intrinsic value.

But there are five companies Philip likes even more for new money. If you need some new stock ideas and want to know what Philip thinks is cheap, you can see those picks right now with a 30-day free trial to Inside Value. There is absolutely no obligation to subscribe.

Bruce Jackson owns lots of shares, too many, in fact. Amazingly, none are mentioned in this article. The Motley Fool owns shares of Berkshire Hathaway. Best Buy, Berkshire Hathaway, and USG are Inside Value recommendations. Best Buy, Staples, and Berkshire are also Stock Advisor selections. The Motley Fool's disclosure policy is a compelling read.