While I am not a professional writer, as in I do not make it for someone else within a deadline rush, I do write my own ideas, with the hope of filming them.
I think it is easier to sit around if you know there is no boss behind you or your meal doesn't depend on what you write so keep this in mind that it also affects hobbyists.
When I have a concept idea that I think is worth the time to be written down I think of it a lot. I mean a
lot. In the bus, in the work breaks, in the subway, at the toilet or when my gf is telling me a long story about her friends etc. While thinking of it I keep small computer files with ideas, connections, good plot twists and everything that could be added to the story and can add a flavor and quality to it.
This is what I call the digesting part of a concept idea. The more you digest it the better it gets at the end, as you already refined it in your mind and on paper many times. You already fixed the errors and added the main good ideas to it.
Next comes the working part, that hard part. I must admit that I don't like this at all, I find it the hardest, as most of you for sure. So I write 5-10 draft pages for example and let it sit for a day. The next day I most of the time rewrite 60% of it. Don't know why, it's just how I work, but I never get it right from the first try. The next day I rewrite 30% of it and so on.
After all the above process I come and clean it and get something that I am extremely please most of the time and that I consider 99% done (as in I have every wording, every dialogue etc exactly as I want it to be).
So I guess that my "secret" would be that I "digest" the idea so much that I explode with desire to write it down. Just like not having sex in a long time
But, this is from a hobbyist, not a pro that needs to eat with what he writes. But note that as a hobbyist you may want the work to define you more, to be perfect while with a job you most of the time want to please someone else and be done with it and go cash in.