Took me a while to get the habit to stick, and what made it work was to find a place and a time. The place had to be different, and the time repeatable. At the moment it's "Monday after work in a coffee shop" — I don't usually go to coffee shops, so it's "different" for me. I turn off the wifi, and give myself an hour.
I stick to it every week, even if sometimes I only move some words about, or rant something unpublishable. But that "80% of the work is showing up" thing works for me.