In many Sports, the intention is to get the ball safely from one end of the field to the other. Although sometimes one player is capable of getting the ball there, successful teams usually pass the ball from one player to another. When one player becomes less effective at moving the ball (either because they no longer have a clear path or the opposition builds up against them) they get rid of it, making sure the ball goes to whoever seems to have a clearer path before them.

Keeping a listener's attention from the beginning of a song to the end works the same way. While it is possible for one lead instrument to maintain focus and attention all the way to the last note, usually you can have great success if you let that focus be passed to other instruments and then brought back to the main lead.

This passing of attention can be as simple as pushing a lick or fill that plays between vocal lines or as complicated as orchestrating an entire section. If you do this successfully, people will happily follow without even realizing all the movement that is happening. For example, many people do not realize how many solos occur between vocals in Pink Floyd's "Money".

In fact, shifting attention away from an instrument even for a moment can make that instrument more compelling when it returns to focus. You can say that this appeals to short attention spans, but I believe that the effect is similar to what can be achieved by editing cuts into a film rather than staying on a single shot. Of course if the actor is compelling a single shot CAN work, but even if you shift to a different view of the same actor in the midst of the same action you will strengthen people's attention and even allow them to reinterpret what they see rather than just continue along.

If you find yourself singing all the parts of a song rather than just the vocal, that song has succeeded in passing your attention from part to part and getting you safely to the end of the song.