People are motivated by the prospect of reward and a reward which arrives sooner rather than later. Creating a good piece of writing under a supervisor or an editor is a strange experience. The immediate reward is the attention paid to the writing. However the most likely consequence is being sent back to do more re-writing. If the writing has been difficult to create in the first place this can feel more like a punishment than a reward as the ultimate reward (publication or other recognition) is still so far away as to be invisible. Motivators suggest self rewarding at visible future intervals such as writing for 20 minutes and rewarding yourself with a 5 minute break. So, really the secret to completing a long task is to ignore the wood and concentrate on the trees.
What do you do to keep up momentum when you are writing?