This little tip has really helped me, especially on first drafts: Set a timer for 25 minutes. Try to be ready to go before you press the begin timer button. Then focus and keep working for that 25 minutes. Then take a 5-10 minute break. Do whatever—a little exercise, check email, surf around, go watch TV, talk to the dog, let the dog out—and then sit back down and write for another 25 minutes. Stay focused during that time. Don't answer the phone or check email etc...When you're writing, write. At the end of the 25 minutes take another 5-10 minute break. DO whatever you want. Then write another 25 minutes.
Even if you only do 2 or 3 sessions ,you will get a lot of words on screen. Give it a try. Hope it helps.