The most important key to developing effective theater games is to get your students to concentrate on the motivation of the characters they portray.  Try to convince them that the important thing is not the character's voice, not the character's costume, not the character's mannerisms, but what the character wants.  Teach them that each character in the scene must have a strong motivation, and that the motivations of the characters must conflict in some way.  Stress that, when on stage, you concentrate solely on getting the thing that your character wants.  When you get it, or when there is no more chance of your getting it, then the scene is over.  Stress also that the thing that makes scenes work is conflict.  Remind them over and over that WITHOUT A CONFLICT, THE SCENE WON'T WORK.  This is a hard idea to get through students' heads, but once you succeed, you'll get some wonderful scenes.