I just saw this tody on Make. They have a video of a guy explaining how he ties his canoe to his car.
Basically after the canoe is on the car, you tie a rope to one side of the bumper or one of the front toe-hooks, thread the rope through the loop on the canoe, and thread the rope through the other side of the bumper or toe-hook. Then you tie a loop in the rope between the last toe-hook and the canoe, bring the end of the rope through the loop, and pull the rope down using the loop as a pulley. Finally tie a cows hitch around the doubled rope to secure it.