My writing output has been increasing lately because of this. When I’m not writing, I try to think about what I’ll write next. Simple right? But it’s so easy to get distracted, and busy, and lose sight of what you were doing. Especially if you don’t have much time to write, you’ve got to make the times that you can, count.
What is your next scene about? What will happen? What does the setting look like? How will this affect things down the road? How can you make it scarier/funnier/more romantic/more thrilling etc? All these thoughts can be running through your head while you wash the dishes or walk the dog.
Then, when you finally have a few minutes to write, you’ll know what you have to do and can get the words out as quick as possible.
Like anything you want to be good at, it has to be a focus in your life. Most of us don’t have a lot of free time, but thankfully a lot of the work of writing can be done right in your head! Get started!
Jonas David 
Well said