Limit the number of people in a group. The problem that most students have with group work is that they feel the workload is unevenly distributed among group members. One way to ensure that each group member contributes equally is to keep groups small, which results in less anonymity for slackers, and a small group requires less time to distribute work among group members. Even splitting students into pairs can be an effective way to teach group effectiveness. Two students can engage each another intellectually, and each student can benefit from the other person's perspective. Pairing also eliminates the possibility of one student feeling like a third wheel in a three-person group.
Grant each group leader some accountability. Often a leader is necessary to guide a group toward a task. Granting a leader limited power to reward and reprimand group members allows each group to develop autonomy. A group leader is more heavily invested in the project, and the leader gives the rest of the group somebody closer and more simple to relate to and answer to than the teacher. Group leaders develop responsibility and management skills in addition to the normal lessons provided by group work.
Separate lone wolves. Some students are extremely resistant to group work; they believe they are intelligent and motivated enough to do any task alone and think that a group will only drag them down. These students are called lone wolves, and they don't fit well in conventional groups. Placing all the lone wolves in one group can be surprisingly effective. They still want to prove themselves, but they settle into each other's company very well when they recognize their mutual qualifications. Some lone wolves may even warm up to the idea of group work and resist it less the next time.
Quiz students in groups. While quizzes that account for a small portion of the overall grade can produce the effect of keeping students interested in the class material, quizzing groups of students may improve that effect. Students who must answer questions as a group for a shared mark work harder individually so that they don't bring down other members of their group. Even students who show a pedestrian interest in coursework study harder when they know other students depend on them.