Look at the deadline of any new assignment you get and write it on your calendar. For assignments with no deadline - a frequent phenomenon in graduate school - impose a deadline yourself.
Imagine how the world will look on that date, to make it real to you: What will the weather be like? What else will have changed? Don't allow those changes to pass without finishing the assignment.
Calculate roughly how many hours the assignment will take.
Decide, before bedding down each night, how much time you're going to spend the next day on the assignment.
Remind yourself, upon waking up, of this amount of time.
Tell yourself that you won't allow yourself any leisure activities that day until you've put in the promised amount of time.
Reward yourself, when done for the day, in whatever way you see fit. This reward is key - having a positive incentive can make all the difference.
Don't be afraid to call your student services office. There are well-trained people out there who can help you.