Pascal said that we enjoy music because it causes us to count without us knowing it. I believe this is also what causes music to increase focus and creativity. Even if we are not paying close attention, our brain counts each note and change of rhythm. This counting occupies the part of the mind that hinders our creative abilities. Like a soothing lullaby, music puts the worrisome unfocused part of the mind to sleep so the productive side can get to work.
Music can also have a profound effect on mood. If I am in a bad mood at work I will put on some music and get down to business. It always takes my mind off what I am doing and makes me a happier person. It depends on our particular tastes, but listening to music with soothing rhythms and a positive message helps us forget our work and think happy thoughts.
For focusing on a creative task, I have found that listening to music without lyrics is most effective. There are no words to abruptly grab your attention and instrumental music tends to have the most compelling melody and composition. The dashing enthusiasm of Mozart or Beethoven enhances the speed and agility of creative thought.
If I have trouble concentrating because of outside distractions or my own scatterbrain, I put on some classical music. This relaxes my mind, blocks outside noise and helps organise my thoughts. Mozart works very well and I have heard this is because his rhythms resonate with human brain waves. Whether this is true or not, classical music is a great tool for increasing concentration and productivity.
The results we see will depend heavily on our personal tastes. Experiment with the types of music you listen to during certain tasks. Through trial and error you will eventually discover your optimal soundtrack. It might not seem like a big deal but listening to music can change our state of mind, causing changes in mood and productivity. Using music as a tool is an easy thing anyone can do to make themselves smarter and happier.