Look for some of the reasons for lack of motivation, including low self esteem, lack of home support or even low expectations in the classroom. Maybe a student needs glasses and doesn't realize this, causing her to avoid completing work and making her appear unmotivated. Not all students who lack motivation are simply lazy. Finding the root of the problem can help you better address the problem.
Engage those students in the classroom who you feel are simply not motivated to work. Rather than allow these students to sit and think about other things, call on them to answer questions. An unmotivated student's mind will wander at every opportunity. Challenge the students who lack motivation to participate by drawing them into the lessons. Don't allow them to disconnect.
Reward students with praise aimed at boosting self esteem. Be careful about offering physical rewards. Physical rewards can be, and often are, used to encourage students to complete work. Use rewards occasionally while stressing that the same effort should be put forth when a reward isn't available. In some cases, unmotivated students will do the minimal work necessary to gain a reward. The motivation this creates is only temporary. The motivation created by constant rewarding is motivation for the reward, not motivation to learn.
Build on the individual strengths of your students. Take these strengths and weaknesses into account when seeking a way to teach each student. Students don't learn at the same pace and don't absorb information in the same way. Recognizing this and tailoring your approach to students who learn differently can help engage those who might otherwise be lost.
Set realistic goals for students who show a lack of motivation. Setting smaller goals may make it easier for these students to accomplish them. This can keep students from feeling overwhelmed, and the small accomplishments can create a sense of achievement that often translates into a greater motivation toward learning.