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NEW YORK ( TheStreet) -- There's nothing worse than watching the averages roar higher while your portfolio just sits there, Jim Cramer told "Mad Money" viewers as he devoted his entire show to teaching investors how to best take advantage of short-term rallies. Cramer said the most important lesson in dealing with big market moves is preparing for the future and not letting great opportunities to sell pass you by. He said just as investors can't give in to despair when the market plunges, they also can't get blinded by euphoria when things are going well. "You don't actually have a profit until you sell something," Cramer reminded viewers. You aren't making money until you "ring the register." That's why selling into strength is always the best policy. There's nothing wrong with feeling good about a rally, as long as it doesn't lead to complacency -- the enemy of every investor. You can be thrilled about your portfolio's performance, said Cramer, but don't forget that you've also been given a chance to sell at great prices. Remember the goal of investing: buy low and sell high.