Ted Nugent's Catch Scratch Fever is worth 4 mph.

I picked up an mp3 player a while back. It's about the size of a 50-cent piece, clips onto the inside of my cycling jersey and has ear buds. It helps to break up the boredom, and it seems to boost my cadence. I pop out one of the earbuds, or shut the thing off, when necessary. I wouldn't think of using it in moderate to heavy traffic. I have a nice commuting route, however, that helps me avoid that to a very large extent. I also ride with mirrors, which I frequently check.