The natural tendency to remember unfinished things is called the Zeigarnik effect. Eve notice how some television commercials get cut off early? The company paying for the commercial cuts it off so that it sticks in your head longer than other commercials. The best way to forget unfinished thing (commercials or songs) is to finish them in your head. If a song gets stuck in your head, trying singing the last lines to yourself. You’ll be amazed how quickly it goes away.