I have trajillions of bad writing habits, but here I will discuss two of the most mundane. They are not habits of craft or production. But they are really unfortunate.
1) I write best when I am also drinking a diet Coke. I try so hard to eat organic, nutritious things. I don't consume any other artificial sweetners. I'm a vegetarian, for heaven's sake! Yet, when I don't know how to link one scene to the next, or I can't nail a patch of dialogue, cracking open a can of that fizzy artificial goodness can cure all ills. I've given up trying to use mineral water, etc. - it has to be a diet Coke.
2) I do not write by typing words on my ergonomic keyboard while sitting on my back-friendly Swedish office furniture (not that I own any such furniture, anyway.) No, I sit hunched over my laptop, on my couch, with my feet up on the coffee table. I'm pretty sure my spine will someday curve into a perma-C from this abuse. Again, if I could manage to be creative while maintaining good posture, I'd do it. But I can't. So . . . meh. Moderation in all things, right?