When your concentration begin to wane, tell yourself to work for 2 more minutes or complete 2 more tasks. Once completed, settle on 2 more.

When you break up a task into tiny sections, you'll find that you improve your concentration. It allows your mind to focus on smaller portions of tasks, thereby seemingly decreasing the amount of work that needs to be done. You can use "Two More" or "Five More" or "Ten More" or whatever works for you, but make sure that it's a number you can work with.