I sing song chains - pick a long song, then find a song that has a key word from the first song, then sing a song that has a key word from THAT song... and the other thing I do is keep the radio on SCAN, so that I can't accustom to it since it changes every few seconds. I'll take it off SCAN to hear a song I like, then click it right back on again. But then again I love driving, even in Ireland on what was, for me, the wrong side of the road...