If you feel tired, easily distracted or just not focused, you can use a "work sprint". Instead of just seating in the cube trying to be focused, start a 20 minutes or 30 minutes of pure work time. Then give yourself some prize (coffee break, surfing the net ...).
I use 'Pizza Timer' freeware to show a timer that go backwards. It is just great freeware and help me to stay focused on hard times (like after lunch).