The Easiest, and Most Developmentally Effective, Way to Team Teach

Team teaching is often, and rightly so, thought of as collaborative teaching. When time permits, and team teachers are willing, planning the lesson can be collaborative, too. But usually, time is a constraint.
For many reasons, the most successful approach is for a JTE or lead teacher to determine the lesson flow and content over the days, weeks and months. Then, the JTE asks the ALT to regularly prepare and lead activities, or lessons (as the ALT demonstrates ability and competence), and to prepare Lesson Plans. The JTE also needs to prepare Lesson Plans for the ALT so that he or she knows what to expect and do during a class.
In this way the JTE, the teacher who is always in the class, maintains lesson continuity. And by taking turns in preparing and leading activities or lessons, the ALT and JTE begin to learn the styles, and strengths and weaknesses, of the other teacher.
One added advantage of this style is that for an ALT who visits many schools, successful activities and lessons from one school, and from different teachers, can be shared with other schools.