I consider myself to be an effective classroom manager and one of the things that has helped me tremendously is using a timer to keep students on task. I use a timer all day long. I time the students as they transition from one subject to the next and we even create time goals to see if we can "beat" our record. We try to do things in the shortest amount of time possible so that more time can be devoted to instruction.
I give students time limits for classroom work and I usually set the timer on the desk of a student who has trouble focusing. I announce that he's the time-keeper, but in reality, the timer is on his desk to keep him on track. I'll usually whisper a directive in his ear like, "I need you to make sure you've completed five of these problems within two minutes" or "When the timer gets to 5:00, you need to be finished with this side of your paper and moving on to the next." This strategy has been quite effective with this young man.
In addition to using a timer, I also use music to set a time limit for an activity. For example, when the students pack up at the end of the day, I'll occasionally play a two-and-a-half to three-minute song and they must be packed up with "nothing on top of the desks and nothing underneath" by the time the music stops. I've seen my co-workers use music in this way as well. A former 5th grade teacher at my school would play the "Mission Impossible" theme when all the students arrived. The children had until the end of the tune to get everything ready for the day (e.g. notes for the teacher and homework turned in, books/paper/pencils out, etc.). She would also play "Takin' Care of Business" at the end of the day during pack-up time.
Using timers as well as music to set a time limit has been helpful in making my classroom run more smoothly.