It is easy to head into a rehearsal setting without a clear plan. I have found this can often lead to disorganized rehearsals. It can often take quite a few minutes for the ensemble to even play a note together or it can lead to a plethora or aimless runs of a piece.
Since I began studying at the University of Iowa, organization and time management has taken a priority for me. Every morning I write down exactly what I need to do for the day and check things off as I complete them. The same thing should be done for a rehearsal. Make a list of things you want to accomplish in that rehearsal. Maybe the goal IS running the piece, but having it written down will make the focus more clear and there will be no time wasted at the beginning of rehearsals.
One of the many things I enjoy about performing in the Wind Symphony under Dr. Heidel is his organization. Every day I enter the rehearsal room, there are sections of pieces and times written down on the board so there is never a question of what the goal of a rehearsal is. This can also lead to better structure, if you decide that at 2:15 you will be switching gears to another section or piece, it can get the whole group on track.
While it may not be practical to plan every minute of every rehearsal, I encourage everyone to at least have a concept of what they want to accomplish at the beginning of each rehearsal.