...Then I asked Mr Kahne if he could explain the method by which he has trained his memory. "Well, in the first place, most people have a wrong idea of the faculty commonly called memory. Some regard it as a sort of adhesive jelly upon which facts will stick and remain until called for. Others look upon it as sort of card index where thoughts are sorted, to be retrieved at will by pulling a sort of mental ‘tag’ --- the ‘tag’ being what is commonly called ‘the association of ideas’. But such methods of memorizing are automatic rather than systematic."
"Then what is the secret of remembering several things at a time?" I asked.
"Focus", replied Mr Kahne promptly. "If you take a camera with a new roll of film and expose it five times at random, you get five blurred images. But if you focus the camera carefully upon a given object and then make the sixth exposure, you get a distinct image. So it is with the brain.
"It is often said that half of us go about with our eyes shut. It would be more correct to say ‘with our eyes unfocused’. And the same applies to our ears. We see without observing and we hear without listening. Most people could, if they tried, train their brains to hold five times as many impressions as they do at present. When they are forced to focus their senses, they do so with the greatest ease. For example, of you carry on a conversation in a crowded room there may be several people talking at the same time, yet you will have no difficulty in fixing your attention upon the words of the one person who is talking to you. That is what is generally called concentration."
http://www.rexresearch.com/kahne/kahne.htm