A quick writing tip, because I’ve just been in the situation: When you are about to finish for the day, try to break at a point where you know exactly what the next sentence is going to be. This way you can start your next session on a high energy level and get into the flow much more easily. That doesn’t mean that you can’t stop if you’re stuck though. In the contrary, sometimes taking a step back is the best way to beat the writer’s block. But if you’re on a roll, save a little bit for the beginning of your next session.