There's usually a reason if you can't concentrate. Perhaps you're tired or stressed or the work you're doing is very boring.
Sometimes you just need a break. To shut down for a while. Other times activity and exercise are the key. They help work through the boredom or stress. And then there are times when you just have to force yourself to concentrate. The times nothing helps and you have to get the work done. You need to go over everything again and again until it sticks. It sucks, but sometimes that's the only option.
If all else fails, meth.