I always present material in roughly 15 minute sections, and I try to vary the physical requirements of each subsequent activity. For example, if I do a mini-lecture in which the class is stationary (and listening, knock on wood), then the next activity will require them to move around. This could mean a group activity or case study (in a group), etc. That can be followed by student or group presentations (in which one person presents while the rest listen and respond), etc.
