Figure out your daily word quota. Start by deciding how long your book will be. A word count of 50,000 is about the length of a short novel such as "The Great Gatsby" or "The Catcher in the Rye." Divide the word count by 21 to arrive at your daily writing quota. For example, a 50,000-page book completed in 21 days requires writing 2,381 words per day.
Make a table showing days and cumulative word count. This will tell you that by Day 5, for instance, you will need to have written 11,905 words.
Find time in your schedule to write. Writing 2,381 words will likely take two or three hours unless you're a very fast writer. So, plan to put in the time. Some writers like to get up early and write for an hour or two before work; if you're a night owl, staying up late may yield better prose. Alternatively, carve out several writing sessions during the day or pencil in long weekend writing sessions with short bursts of writing during the week.
Outline your book project if you like, but only loosely. Chris Baty, creator of NaNoWriMo, points out that developing intricate plots and characters can be paralyzing. He recommends leaving most content up in the air, which allows you to improvise and go off on fertile tangents. It's also fine to start writing with no outline whatsoever and see where it takes you.
Don't get behind. When you're writing under a demanding deadline, falling behind is dangerous. Start off strong, completing your full word counts for the first few days. This will give you confidence. Don't let yourself fall behind midway through the project, either. Remind yourself that soon you'll have a completed book project in hand.
Write without editing. Get the words on the page and don't worry about honing your sentences or saying everything perfectly. You can edit and polish your prose later. Plus, it's liberating to write freely, and you may discover that your creativity flourishes.
Tell your friends and family about your plan. Let them know that you're writing a book and encourage them to check up on you. Brag about the project so that you'll feel obligated to see it through. Create social pressure to help you finish.
Reward yourself. When you've completed your daily word count, indulge in a special snack, a bubble bath, a beer -- whatever you especially enjoy. This conditions you to associate writing with a reward. When you complete your manuscript, celebrate lavishly. Writing a book in three weeks is an amazing achievement.