Give positive feedback to the student. You may not know at first whether a student's complacency is born of a sense of inadequacy, so make sure he is given specific praise for the things he does well or for when he improves. When criticism is required, make sure it is constructive and offers a solution.
Create lessons which involve the student to be actively involved. These can include learning games, hands-on experiments or partner-based projects. No one really likes to be lectured, so create a discussion about the subject whenever possible and encourage the student to weigh in.
Listen and then mix things up. Open the floor to your students about ways they would like to see the class led. Consider their suggestions on how to review for tests or what kind of environment they find most interesting, i.e., hands-on projects as opposed to pages of notes. Having the same routine can be beneficial to some students but can cause stagnation in others. Make sure you are also catering to those who get bored easily.
Set goals that will be rewarded when completed. The motivation may need to be for the whole class, such as a class party, or individually, i.e., free time every Friday for those who turn in their homework all week. Take your cues from the students in question. A series of small goals may feel more doable than one large one that may seem too hard or too far away to be worth it to them.
Expect the best from all of your students, especially the complacent ones. Make sure they know that not only do you expect their best work, but that you know they are capable of it. Taking a personal interest in unmotivated students helps keep them from flying under the radar and getting by with "just enough." They also need to know that you care about their education enough to not give up on them or consider them a lost cause.