There are many tips for this situation--maybe one or two will be helpful to you.
First, don't feel as if you have to read every word of the assignment, especially if you usually wind up not reading much at all. Aim to read a few key parts--think of it as a triage approach. If you know you can't or won't read it all, what parts can you read/should you read to maximize your success in the class?
Second, it is very important and helpful to you if you can get an overview of the reading. Leaf through the pages of the assignment, looking at any titles, charts and pictures. Try to get an idea of what the main points of the reading are about--make guesses of what you think you will learn if you read it closely. Assess which parts of the reading will be important to the class, as well as which parts will cover information you already know well. Then, go back and read closely the parts that seem important but not well known.
Third, try to be active as you read, especially if you are easily distracted. One way is to keep a highlighter in your hand and to highlight key phrases here and there as you go along, if for no other purpose than to keep yourself moving through the reading assignment. (It is wise, however, to avoid highlighting entire sentences or paragraphs--just a key word here or there). Another is to use a speed reading technique of holding an index card under each line and moving it along. This method helps you focus.
Fourth, eliminate distractions. Find a good place to read (and a good time of day--don't do it when you are going to be too hungry or tired or excited to focus).
Finally, realize that part of the problem is procrastination. You may be putting off the reading and promising that you will sit down "tomorrow" and do it all (in one sitting! Ouch! that's often a recipe for failure). Instead, try to break it into do-able chunks: for example, make yourself read for 20 minutes a day throughout the week rather than for 2 hours on the weekend.