That's a very interesting question, Andrea.
I think my problem is not so much trying to discipline myself to write as trying to make myself do something other than write! I'm a workaholic really and there's nothing I'd rather be doing. But in the early days it was harder, I think because I lacked confidence in what I was doing and thought it was a bit ,well, mad, getting so involved in these made-up stories and falling in love - no, really - with these heroes that I'd created. (My 22 year old son refers to my writing as "playing with my imaginary friends".)
My books are also quite ambitious and I didn't know if I could actually do the things I was trying to do, so I think I often needed a confidence boost and an energy boost. I've always used music for that. (Springsteen is great for making you think you can achieve anything!) I have a kind of playlist for each book. I lot of writers do this. Music will take you directly into the world of the book the way scents allow you to access memories.
The other thing I do which helps me get into the world of the book and get down to work quickly is that I collect pictures - mostly photos of people - which represent places and characters in the work-in-progress. So above my PC there are a lot of photos clipped out of magazines or printed off the internet representing the characters in my current book. A character doesn't really take off for me until I have found a visual real life equivalent. Their personality doesn't have to resemble my character's, just their face.
I teach creative writing occasionally and I find that mostly what stops people writing is fear. Fear that it won't be "good". But if you stop trying to write "well", but just tell the story, the writing flows more easily. You have to get that critic off your shoulder, the one who sneers, "Whatever makes you think you can write?" I don't ever worry about writing well now, I just try to say what I want to say, in the way that I want to say it. For me, that's good writing. :-)
Linda