When it comes to front ensemble warm ups, I usually sit down and make a list of every thing that the students do during the show; from double stops to independent rolls. Then I make sure that when I write the exercises, I include these elements in the exercises so that way each concept is covered when they are done warming up. Also, I sometimes take some of the more difficult sections of the music and modify it to where its a warm up. I agree with msach about scales; I usually just teach the scales that are used in the show.