Most human beings like routine as well as variety.  They like to park in the same place, sit with the same people, travel the same routes, and organize their life in a pattern.  In the meantime they talk about and often seek variety to break up the routines of their lives.  This creates a balancing act for instructors.  They need to establish a routine for getting classes started yet provide instructional variety to keep students engaged.

One way is to establish a set routine for starting each class.  This way the students will get settled into their seats or lab locations.  They know announcements will be made and then the next phase of that day's course will start.  This lets them relax, catch their breath and then mentally prepare themselves for learning.  This is when you can bring variety to your instruction.  You can break them into discussion groups, have a guest speaker, do problem solving or lecture.  The point is their minds are prepared after the routine phase of the course to accept variety.  In fact they prefer variety because it will keep their attention while helping the time go quicker.  This leads to effective and efficient learning.