I was suffering from lack of concentration later in the workday, until I stumbled upon the Pomodoro focusing technique. I've been a lot more productive and in a better mood since I started using this two weeks ago:
The Pomodoro Technique is a time management method developed by Francesco Cirillo in the late 1980s. The technique uses a timer to break down work into intervals traditionally 25 minutes in length, separated by short breaks. These intervals are known as "pomodori", the plural of the Italian word pomodoro for "tomato".
The method is based on the idea that frequent breaks can improve mental agility.
I created an AutoHotkey script that uses timers and a global hot-key I set for WinAmp to automatically stop and start the music after instructing me to work / have a break. It works really well for me.