Rewards for work done. If you are finishing your current WIP, whether that means writing more on it, editing or proofing for the umpteenth time, then your schedule should have a set schedule.
"I must write XXX words today before I am allowed to deviate and fiddle with Project 3," or some such.
That way you are making progress on the one you need to finish, rather than having a garage full of partial/first drafts, with fifteen projects scattered on your screen that you are currently working on, knowing in the back of your mind that, soon, you're going to have to rent a storage building because you can't even park a Smartie in that two car garage anymore...