Thinking. Daydreaming. Imagining. Reenacting. Always priming the pump.
Often, when we sit down to write we put our fingers to the keyboard and expect the story to come out without doing the legwork to bring the story about. Are you constantly thinking about your characters so often that you know them, that you know how they would react in a scene? Do you know what your next scene is? Have you been replaying that scene over and over in your brain, mulling on the words spoken, getting it just right? Do you go over your themes in the shower? Do you wake up thinking about how your characters would wake up? Do you ride the train coming up with all the possible plot points that could happen in your novel? Do you wait for your coffee to brew and go over that scene again, the one you love, the one you can't wait to write?
If you do this, if you use your waking/eating/commuting/shower/wait time to dream forward, then when your writing time comes, you won't be at a lack of things to write. Sometimes I think we don't think about writing at all until we sit down at the computer or the notebook or the typewriter, and then we expect it to flow. It would be like someone showing up at the Olympics ready to sprint without a day of training.
Train your novel in your head. Practice all the outcomes, play with the game plan, push your characters. Then you'll never be at a lack when you sit down to write.