Break it down.
The day you get the paper assignment (ideally), or shortly thereafter, break the writing assignment up into the smallest possible chunks. By doing this, the paper never has a chance to take on gargantuan proportions in your mind. You can say to yourself, “Right now, I’m going to write the introduction. That’s all, just the introduction!” And you may be more likely to sit down and do that, than you will to sit down and “write the paper.”
Get a new attitude.
We shoot ourselves in the foot, to begin with, by telling ourselves how horrible a particular writing assignment is. Changing our attitude toward the task, when possible, may go a long way toward keeping us from procrastinating. Tell yourself that the task isn’t so bad or difficult, that you either know how to do it, or that you can learn how while you’re doing it. You may find, too, that if you start early on a particular assignment, your attitude never has a chance to get very negative in the first place! Simply starting to write can often help us feel more positive about writing.