Clownac makes a good point to. There are a variety of things that keep you proactive on you writing development.
Only tip I can give is have a goal/purpose/direction so you never fall into a I'm stuck circumstance. Because everything you choose to add or not are the details of your story that give it personality and establish your mark as an author.
I contrary to clownac section my works. Making my story as a series of episodes. This allows me to have fun and choose the next scene based on the limits/laws of my world.
Being half through something makes me ponder longer on how to go about it stressing me and prolonging my advancements. I prefer to end at the start of the new scene.