My friend King Siddharth wrote a blog post on focus: http://www.64notes.com/focus/
He has an interesting take on focus. Its not about narrowing down your activity sphere, it is controlling your emotions. I think that can be the key thought you can take away. Its not about the subject at hand. Its about controlling or conquering your emotions which might distract you from your purpose.
My trick for focus is this -- do something small and bite sized for 20 minutes and then focus kicks in. Slowly the juices start to flow. So the first 20 minutes is the pain.Choosing WHAT TO FOCUS ON is the hardest battle.