"But note this important fact: The true investor scarcely ever is forced to sell his shares, and at all other times he is free to disregard the current price quotation. He need pay attention to it and act upon it only to the extent that it suits his book, and no more. Thus the investor who permits himself to be stamped or unduly worried by unjustified market declines in his holdings is perversely transforming his basic advantage into a basic disadvantage. That man would be better off his stock had no market quotation at all, for he would than be spared the mental anguish caused him by other persons' mistakes of judgement".
This is probably one of the greatest statements with regard to investing that has ever been. I could spend days writing about Ben Graham, however you would be much better off just reading all that you can about him and his investment strategies. In case you don't know, he was Warren Buffett's professor at Columbia Univeristy. Do I need to say anymore?