At first, I wrote for 60 minutes every day, doing freewriting with a bit of editing or just editing. I tied that to the end of my morning ritual (before checking email). Then I decreased the minutes to 30, because it was sometimes painful. And then I've written only on some days. Frankly, I'm not sure why. It had to be fear of some kind. To finish the book, I've worked with an editor - that has really helped me to push it to completion.
Now I know 100% that I can do it, and that I can give tremendous value to the world through my writing. This should help with the resistance; but it's probably not all there is to it.
I think writing daily is the best way - even if I would use the time just for research and capturing a few ideas. I always like to have a main goal/focus.