It used to be a big problem for me but this year I've had so much homework to do in little time constantly, I kinda had to learn to stop procrastinating.
Don't do your work in your bedroom because you'll remember how you relax in there and you'll be in a non work mode from the start. Go to the kitchen or a library or something where you don't normally relax. Set mini deadlines for yourself, like 'finish 5 questions by 2:30' just little things like that rather then tackling the entire thing at first. Do it in small steps and take breaks in between (not too long though) so you don't get overwhelmed. Don't be in a room with a TV that could distract you. If it's a non computer related assigment, don't be near a computer either. Don't even have it in the room to tempt you. If it is a computer assigment, use a program to block websites that you'll want to go to to have fun. Don't start the assigment really late at night/the end of the day if possible, do it earlier in the day when you won't feel as rushed or tired.
