If there is one thing that works to keep me motivated it is deadlines. If I know I have a commitment to accomplish a task by this time I will keep working until it is done. While most of our experience with deadlines is external, school or work telling us when things are due, there is no reason we shouldn't, and may reasons we should set deadlines for ourselves.
First off deadlines keep us focused. The knowledge that task X must be done by time Y is a great way to keep it at the front of my mind.
Second they can help fight procrastination. Just because a project comes with a built in deadline does not mean we cannot move that up. What to make sure you get your homework done and can still have fun over the weakened? Set the deadline for the paper due Monday to Friday and get it done early.
Third setting a time frame for the completion of a task can create the pressure some of us need to work. Without a deadline a goal is more of a dream, something that will happen someday. With a deadline it becomes more tangible.
For those of us who hate deadlines it is more the fear of missing them then the actual deadline that causes us stress. If we move the focus off failure, in other words stop worrying about making the deadline we can instead use it to set a timeline for completion. For example if I know I have to have a essay done in the next five days and I have to watch two movies to compare and contrast in the essay then I know I need to watch at least one today and one tomorrow so I have time to digest them before writing.
Deadlines are tools we can use to keep ourselves on track. If we do not fear them, do not cause ourselves undo stress over them they can help us get done everything we want.