I've already covered priorities and breaking up tasks. Once those are in place, it's easier to make time to write. When I'm doing my daily tasks, the kids are with me, doing their jobs and helping. Once the work is done, I reward them with "TV time". They can watch a show for a little while and I can get some writing in guilt-free. If your children nap, this is also a fantastic time to write. Your daily tasks are already done (priorities) now sit back and write while the house is quiet. But...if you're like me, my children don't nap :P They all stopped between 18 months and age 2, so that's not an option in my home. If your kids want to be near you while you write, have a box of special toys that you only pull out during writing time. Let them play and explore with the awesome new toys while you sneak a few extra minutes.
You can also use all the "dead moments" in your busy day to work on outlining, brainstorming, marketing, or free writing. For example: waiting at the doctor's office, waiting in line to pick up kids from school, while in the shower, etc. If I spend my free moments thinking about my current book, by the time I sit down, I already know what I'm going to write, so I don't waste time sitting and pondering.
After bed is my typical writing time. I try and get a couple hours in once kids are all kissed and tucked in. And since I've spent the majority of the day already marketing and brainstorming my next scene, I can just sit down and take off.
Even when you have packed days and busy evenings and it seems nearly impossible to write, never stop. Write a little bit every day. Even if all you get is one sentence, that is still progress! And in the end, you'll end up with a book!