I've used extra credit to motivate students to study at home. I give them study questions to help them find specific information that I will emphasize at the next class period. One quarter I decided to give students extra credit for completing the study questions. I quickly discovered that the students who would complete the homework without being coaxed were those that got the extra credit points and those I wanted to motivate did not comeplete the homework regardless of the extra points.I never offered extra credit for homework again.