I am writing a Young Adult novel, and it's been in the works for about a year. Why so long? Well.... I write sporadically. If you are writing a book, you are probably writing sporadically, too. Here and there, when you can. Which isn't much.
Thus, my Writing Challenge! Write 500 words a day. That's it! Just sit down and type--no editing allowed. You'll be surprised what you can accomplish. Do it starting today until Friday. That's only five days, but times that by 500 words and you'll have 2500 words.
Update: I did three out of the five days. But on one of the days, I did 1,000 words! When you sit down to type, it seems like more just comes out than you expect. Which is why it's good to sit down often.