Writing is a muscle memory that needs practice to work, but if all you do is write when you have nothing to say, you spend all that hard work learning how to say nothing.
Flipping the problem around, what I think you need to practice is learning how to feel motivated even when you don't feel like writing. Each scene of a story is a car on a box that does something for someone. Some cars are engines just meant to pull the story at the start and push at the end, but each is a discrete unit meant to carry something to the reader.
Looking at at why you don't want to write is far more valuable than forcing yourself to write when you're not feeling it. Is the scene that you're trying to write relying on a character acting out of character? Are you bored with what's going on? I've read so many books where the characters talk about what they're going to do for a chaper, do the thing in chapter two, talk about what they have done in chapter three, talk about what they're going to do in chapter four, and actually do it in chapter five. Now you're 10% done the story and all that has happened was two chapters worth of plot.
If your life needs your attention, whether it be work, school, family or just some personal things you have to deal with, go deal with the problem. Writing is important, but it's not as important as your future or the people you love. Don't beat yourself up if your life has become a riptide, pulling you away from your hobby. If you fight it, you'll drown. Just keep swimming until you're not being sucked out to sea, then make it back to the shore and keep writing.
If you do have time and you still don't want to write, make the worse possible thing happen in your story. Kill the guy they needed to talk to, make the MacGuffin fake, cause the earth to open up and swallow half your party. Whatever it is, make your characters react to the new situation as well as the plot line (then go back in draft 2 and foreshadow the explosion, but that's for another day).
If that doesn't work, it's time for more drastic measures. Go back to the last pace that you felt excited about writing. If that's 40k ago, off it goes. If you're not feeling excited, no one else will. It's more work to fix dispassionate prose than it is to entirely rewrite the idea from the ground up. If you've never felt passionate about that story, start something new.
No one is waiting for more drab words on a page. The first reader wants to get off of slush duties and start their career as an agent or editor who takes clients out for lunch, not brown bagging it while reading the slushpile. You're never going to win them over by producing work that you wrote when you'd rather not be writing. She needs to convince her boss to love it, who needs to convince her boss to love it, who needs to convince an editor to love it, who needs to convince they're acquisition board to love it, who needs to convince marketing to love it, who needs to convince the book buyers to love it, who needs to convince the book buying public to love it.
If you're going to pick up a new habit in 2016, have it be learn how to love what you write and write what you love, not produce more of what you don't. Most of us started to write because we were entertaining ourselves. When you're trying to train a horse to do something, if you're bored with it, the horse has been bored for the past five minutes. If you're bored with what you're writing your reader has been for a while.
Writing is about butt in chair, but the world doesn't need more stories that feel like an obligation. Realistically, most people won't ever write at a level to support themselves on it, so even a moment spent forcing yourself to do something you don't want to do is too long. If you have the motivation to write, you won't need discipline.