Point out your students' abilities and encourage them. For example, if you notice that one of your students is talented in critiquing the books you assign him to read and that he has an unusual maturity for his age, tell him that he has good analytical skills and he would make a good researcher or lawyer one day. Also tell him how he can prepare for those careers.
Find out why your student lacks motivation. If a student tells you she has not been motivated to learn because she is not sure if she'll ever be as smart as the other students, tell her that not all students grasp the lessons the same way and that you believe she is capable of succeeding despite the difficulties she is facing in class.
Add variety to your class instruction. Instead of giving your students mundane homework assignments every day, change to give them a special project that is fun and easy to complete. For example, if you are teaching physical education, ask each student to create a fun but decent dance routine that the class can use as an aerobic exercise during the following week of class.