When I was a kid, I had difficulty staying on the same task. My daydreams were (and still are) incredibly vivid. I had a lot of physical energy, and I wanted to run around as a kid. And when I can't, being stuck inside studying, I'd have near lucid daydreams of running around outside. Combined with being able to pick up many things up, I become more and more intolerant of my own mistakes and challenging situations. As I entered my early adulthood, my experiences of life started narrowing down as I avoided challenges. And when I try taking on those challenges, I'd run straight into them.
It was later in life that I learned the methods for concentration practice. There are no shortcuts to concentration. There are no clever hacks. Searching for a shortcut is precisely why most people have difficulty concentrating. Your mind does not want to accept what you are doing here and now. So you think you have to "force" yourself to concentrate, and end up burning a lot of energy keeping you on track. This is NOT how you concentrate.
Concentration grows a little bit each time you practice, and the more you practice, the more you learn about how your mind works.
Growing the raw skill of concentration as you learn your primary art is itself the reason other skills take years of practice to master. It is also why, without practice, talent does not blossom into mastery.
Someone asked me to post up a cheatsheet on how to practice concentration; maybe other people here will find it useful: http://www.quora.com/Meditation/Whats-a-nice-little-cheat-sh...