Rule No. 9: Defend Some Stocks, Not All

Editor's note: Jim Cramer's new book, Real Money: Sane Investing in an Insane World, is available in selected bookstores now. As a special bonus to RealMoney readers, we will be running Cramer's "Twenty-Five Rules of Investing." For more about the new book and to order it, click here. Today, we present Cramer's ninth rule of investing. To read about his first rule, click here; for his second, click here; for his third, click here; for his fourth, click here; for his fifth, click here; for his sixth, click here; for his seventh, click here; for his eighth, click here.

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When the markets are hard and unrelenting, as this one has become this year, it's important to remember an adage that's well-suited for a battlefield plan but is just as valuable for a portfolio plan:

He who defends everything defends nothing.

When the market's flying and many stocks are in a bullish mode, it really doesn't matter how much you have on, or how many positions you have. The more exposure the better.

But when things get tougher, you have to recognize that many stocks that you bought for better times may not be in good enough shape to rally. You can't own everything you would like to own.

For example, you may have been playing the chemical sector with Eastman ChemicalEMN, Dow ChemicalDOW and DuPontDD because you saw the demand from China. Suddenly, General MotorsGM "blows up" and you have way too many chemical companies.

I like to say, don't defend them all, just defend some. Pick your favorite and defend that. If you try to defend them all, you simply will run out of capital or go on margin before the bottom. You will lose your reserve and not be ready if the market doesn't turn in your direction.

That's why I rank all my stocks at all times for my Action Alerts PLUS portfolio. I need to know which stocks I will defend when things get tough and which I will cut and use as sources of capital.

It's extremely important, say, if you think that the techs are going to start rallying here, that you don't just keep the whole complex. Pick the best stocks, the ones you know you will want to buy if they go lower and toss out the rest.