1. Do Not Edit
Your first draft is just that, a first draft. To continuously stop to rewrite sentences, rearrange paragraphs, and correct typos will stop the flow of your writing. It also creates what is commonly called writer's block. You can't keep turning on and off your flow of inspiration without consequences. When you sit down to write your first draft, train yourself to turn off the analytical side of your brain and just let the writing flow. Never edit as you write.
2. Avoid Distractions
Turn off the television and throw your cell phone out the window. Distractions will only break the flow of your writing, so get rid of them as best as you can. If you live with someone who does not respect your need for distraction free time, then take your work elsewhere. Go write in the car, in a park, or wherever you feel comfortable and can be undisturbed.
3. End in the Middle of a Sentence
When your writing session is at a close, end your writing in the middle of the sentence. When you do this, it makes starting back up again easier. What I like to do is end in the middle of a sentence and the next day I sit down and reread two paragraphs back. I then pick right back up by finishing the last sentence I partially wrote. It helps. Try it next time.
4. Post a Statement of the Book's Intent
It is easy to lose sight of your book's intent or main goal. What works best for me is to write it on a sticky note and post it to the bottom of my computer screen where I can glance at it quickly when I feel myself going off course.
5. Stop and Stretch
When you reach a pause in your writing, stand up and do some simple stretches. I like to bounce on my yoga ball for the length of a high intensity song. It gets the blood flowing and it will reenergize you.
If you want to learn more about writing a first draft, I highly recommend First Draft in 30 Days by Karen Wiesner. I feel that it is the only book you need on writing a first draft.