In my class last year we had a lot of transitions between teachers. It was becoming problematic at the beginning of the year getting the timing of the transitions down. I wouldn't know I had to line up my students until I saw the other classes line up in the hall. I looked for an alarm clock that had multiple alarms on it to help me out, when my son pointed out that my cell phone had six programmable alarms.
I set all six of those alarms to go off 5 minutes before we were suppose to line up in the hall. I explained the procedures to the children and I put the student who sat close to my desk in charge of turning off the alarm when it went off (they also had to make sure I heard it).
It was GREAT. All the children heard the alarm and knew they had 5 minutes to finish up and clean up. None of the other teachers were upset I was running late and the kids were happy they were never late for recess (that use to happen sometimes :)
Make sure if you are observed, and the alarm goes off, that you (or the students) explain to the observer what the alarm is for so they don't think you are getting random cell phone calls during the lesson.