Give Praise.People of all ages respond to praise. And children need encouragement from the people whose opinions they value most--their parents. "Good first draft of your book report!" or "You've done a great job"
can go a long way toward motivating your child to complete assignments. Children also need to know when they haven't done their best work. Make criticism constructive. Instead of telling a third-grader, "You
aren't going to hand in that mess, are you?" try, "The teacher will understand your ideas better if you use your best handwriting." Then give praise when a neat version is completed.
(taken from: kidsource.com)