I've always loved writing (even as a child) but as a teenager I started to stuggle with "getting in the zone" and ended up giving it up. Then, in my last year of high school, I took Writer's Craft, and it was amazing - I actually learned how to put myself in the zone!
The first thing I do is decide in advance that I am going to write, and try to sort of look forward to it, and keep in the back of my head all the things that I like about writing.
Then, when the time comes, I read for 5 minutes or so. I read something that I really enjoy, that inspires me, and that gets my brain going.
Next, I pull out a notebook, and just write. It can be about anything, but generally I pick a random word or phrase (ie bubble) and write something about it - it could be a short story, a song that it reminds me of, rambling, a memory the word reminds me of, just anything.
Yes, it does take me about 10 extra minutes or so, but when I've done those things I am in the zone, and I love that feeling.
Hope that helps you as much as it helps me!