Focusing on your writing does not simply mean to pay attention to what you are typing. It also means:
Turn off the Interwebs!
What does this mean? Turn off your cell phone, close out of Facebook, Twitter, Pinterest, etc. when you sit down to write.
Be sure to have no distraction when you sit down to write. This will help assure you a successful writing session. Often times, hearing the notifications of social media will draw your attention away from what you need to be focusing on...writing your book!
Try putting in your ear plugs and playing some light, instrumental music while you write. It may help keep you from hearing noises that may distract you. It can also help keep your mind clear of other thoughts that can draw your attention away from writing.