I currently have three stageplays, a comic strip, one screenplay and posts on writing blogs (not to mention NaNoWriMo) that I have to stay on top of, and yet my mind gets scattered, although I find reading these links help.
The only real answer here is just doing it. no excuses no rationalizations, no ill write twice as much tomorrow, for 30 days find the most convenient time during your day that you are not going to get interrupted, then spend the first 5-15 minutes getting into the right mindset, read your last paragraph or your outline, fill your mind with your characters and their plight, be in your writing space, not bed. and write for 30 minutes without editing, it is very important that you DO NOT EDIT, just write, if you say the same thing three times, its fine just keep going. then take a break for a few minutes, grab some coffee or tea, then do it again. no editing, just get your thoughts or your characters thoughts down on the page, tell their story, action, dialogue, movement. don't get caught up in the details. Once you have done an hour of just writing, then relax, continue writing, do a little editing of what you've written, rearrange some things, but DON'T delete it all, you can do that the next day if you need to but you should never delete anything without taking a break from it first.
One way to help yourself develop a writing schedule is to write down everything you do, what time you start and finish, and then finding the gaps in those times. When you see how much "dead" or unused time you have, you can locate the common places and set up a schedule.
Actually doing the writing? Well, I agree with Illyena's answer: You just have to do it. Finding butt-in-chair-fingers-on-keys time is half the battle. The other half is getting BICFOK.
I was struggling /stuck on my first book when I found inspiration - it came from this movie "The Diving Bell and the Butterfly" Based on a true story - this is about the editor of Elle magazine who is paralyzed in a car accident. But despite all that, he writes a book. It's amazing / true determination + one of the most inspiring movies I have watched. Now, every time I get stuck, I remember this and it helps me to get back in rhythm. And yes, my book is now published, 400+ pages & even completed my second book and published it.