Normally, I have set times to write over the week, and a routine to sit down. Generally, I need three things to get started writing.
1. a cup of something hot
2. notes or an outline
3. be half awake
I start out either with a sentence in my head, or by googling some concept I'm going to write about. Sometimes seeing images in particular or doing research helps me get into the story more. And when you're in the middle of a novel, it's particularly a good idea to get into the story before writing. I've heard a lot of people say they should just write until something good comes out. It can be useful, but I find research is an easier way for me to warmup. How do you warm up for writing?