Could half the battle of becoming a productive writer be being prepared? That's my conclusion, based on teaching hundreds of students. Most people who freeze up or don't get around to writing on a regular basis have not gotten ready to write ahead of time. You can find your own way to do this, but here's what I recommend for the composition phase.
To set up your writing:
Commit to writing. Give yourself the gift of 15 minutes (or 200 words) a day, five days a week. Set this time aside as appointments that trump all other activity. You deserve it don't you?
Commit to one project. You don't need to write sequentially. You can jump around to "good parts" or places that are working or even write scenes you suspect will be cut. But the words should add to the story you have chosen to write, not be part of another project. (It's okay to work on other projects, but only if you keep your promise to add to the chosen story.) And you need to finish the project you committed to.
Write down what you will work on the next time you write. If you outline, this is already done. If you write by the seat of your pants, this barely intrudes on your creative process (and you are welcome to write something else in addition to what you stated). These words written ahead of time will set a specific goal and get your mind working on the job. Make sure this note to yourself is in one sentence or a few clear words.
"I will write the scene in which..."
"I will describe the place where..."
"I will record the character's reaction to..."
That's it. Do these three things -- set aside a time to write, choose the project you'll see to completion, and give yourself the assignment beforehand -- and you'll be ready to write tomorrow (or the next time you're committed to write). The words might not flow every day, but you'll make progress on your story and get it finished.
You may want to use this approach as the bones for your New Year's resolution. Begin by putting a start date on your calendar.
What should you write? Most writers I know already have a project in mind or in progress, so that may be what you use to try out this approach. If you don't have a project or can't choose between them, select the shortest viable project on your list or write a short story based on an online prompt to get yourself going.
If this is half the battle to becoming a productive writer, what's the other half? These are the questions I get from students:
How do I choose a project?
How do I know what to write?
What about research?
What if what I'm writing stinks?
What about story logic?
These point to aspects of planning, selection, exploration, and revision, and all of them are essential to creating stories that are publishable. I'll discuss these in the coming weeks (and I welcome your questions and suggestions for topics). But, with what is above, I think I've provided enough for you to build good habits of productive writing and to develop a process that fits your time, personality, and style.