Don't. Follow your plan.

The following is an excerpt from The Intelligent Investor by Benjamin Graham:

A serious investor is not likely to believe that the day-to-day or even month-to-month fluctuations of the stock market make him richer or poorer. But what about the longer-term and wider changes? Here practical questions present themselves, and the psychological problems are likely to grow complicated. A substantial rise in the market is at once a legitimate reason for satisfaction and a cause for prudent concern, but it may also bring a strong temptation toward imprudent action. Your shares have advanced, good! You are richer than you were, good! But has the price risen too high, and should you think of selling? Or should you kick yourself for not having bought more shares when the level was lower? Or—worst thought of all—should you now give way to the bull-market atmosphere, become infected with the enthusiasm, the overconfidence and the greed of the great public (of which, after all, you are a part), and make larger and dangerous commitments? Presented thus in print, the answer to the last question is a self-evident no, but even the intelligent investor is likely to need considerable will power to keep from following the crowd.

It is for these reasons of human nature, even more than by calculation of financial gain or loss, that we favor some kind of mechanical method for varying the proportion of bonds to stocks in the investor’s portfolio. The chief advantage, perhaps, is that such a formula will give him something to do. As the market advances he will from time to time make sales out of his stockholdings, putting the proceeds into bonds as it declines he will reverse the procedure. These activities will provide some outlet for his otherwise too-pent-up energies. If he is the right kind of investor he will take added satisfaction from the thought that his operations are exactly opposite from those of the crowd.

Another excerpt

Imagine that in some private business you own a small share that cost you $1,000. One of your partners, named Mr. Market, is very obliging indeed. Every day he tells you what he thinks your interest is worth and furthermore offers either to buy you out or to sell you an additional interest on that basis. Sometimes his idea of value appears plausible and justified by business developments and prospects as you know them. Often, on the other hand, Mr. Market lets his enthusiasm or his fears run away with him, and the value he proposes seems to you a little short of silly.

If you are a prudent investor or a sensible businessman, will you let Mr. Market’s daily communication determine your view of the value of a $1,000 interest in the enterprise? Only in case you agree with him, or in case you want to trade with him. You may be happy to sell out to him when he quotes you a ridiculously high price, and equally happy to buy from him when his price is low. But the rest of the time you will be wiser to form your own ideas of the value of your holdings, based on full reports from the company about its operations and financial position.

The true investor is in that very position when he owns a listed common stock. He can take advantage of the daily market price or leave it alone, as dictated by his own judgment and inclination. He must take cognizance of important price movements, for otherwise his judgment will have nothing to work on. Conceivably they may give him a warning signal which he will do well to heed—this in plain English means that he is to sell his shares because the price has gone down, foreboding worse things to come. In our view such signals are misleading at least as often as they are helpful. Basically, price fluctuations have only one significant meaning for the true investor. They provide him with an opportunity to buy wisely when prices fall sharply and to sell wisely when they advance a great deal. At other times he will do better if he forgets about the stock market and pays attention to his dividend returns and to the operating results of his company.