A while back I came across a great article about musical arrangement on Bobby Owsinski's Big Picture Blog.
He explains that most songs usually have five elements: Foundation, Pad, Rhythm, Lead and Fills that, when used effectively, can make a great song. Any more than that makes it confusing to the listener.
These elements can also be a helpful analysis tool for a musician in the process of arranging his masterpiece or for anyone who wants to figure out what makes a hit song tick.