Create a mock-up of your proposed book. Buy a blank book of about the size yours will be. Create a cover for it--it doesn't have to be a professional-looking design, it just should approximate how you envision your own cover will look. Use the pages to simulate the layout of the text.
Divide the pages into chapters or sections. Make notes about what each section will include. Draw sketches to approximate illustrations, or paste in photos from magazines. The more detailed you make this "blueprint," the better it will focus your efforts toward the completion of the project.
Set a deadline. Be tough on yourself. It probably shouldn't take you 3 years, but you may not be able to finish it in 1 month if you have a day job. Be aware, though, that many people have done just that. Kris Baty, author of the book "No Plot? No Problem," launched National Novel Writing Month in 1999, and now thousands of people participate every year, writing an entire novel in 30 days. The important thing is for you to decide how much time you can spend per day writing, and how long it reasonably should take to finish at that pace. And above all, stick to it. Keeping a deadline is an extremely powerful motivator.
Set up your "office." Wherever you choose to do your writing, it should be a place where you are be comfortable and undisturbed. If you do your writing in the same place every day, your mind will start associating that place with writing, and just being there will help start the flow of creative juices.
Write at the same time every day to help condition your mind. If possible, make it the first thing you do in the morning, to prevent procrastination. Some writers find it works well to write first drafts in the morning, and then revise in the afternoon. Whenever you write, just make certain you eliminate distractions. And, particularly if you are a person whose mind tends to wander, it's a good idea to set yourself a daily quota of words, rather than just adhere to a time limit.
Be willing to do rewrites. There's an old saying among writers: "Writing is re-writing." Quite often, the biggest part of the job is improving what you've already done. You may be able to "finish" a book in 30 days, but it's highly unlikely you'll have a masterpiece. Quality work almost always takes revision.
Look for a publisher--or publish the book yourself--once the book is truly finished.