This is how I started running my practices a few years ago to stay on track and since I do not always have a manager or someone to run the clock.  Captains are responsible for stretching and any announcements they have before practice starts.  It has helped us start on time and make the most of our 90 minutes.  I put 99 minutes on the clock if we are the early practice and the players are always ready to go when the clock hits 90 and more often than not even earlier. The first year it really challenged me more than the players because I had to be more aware of time and trying to be efficient .  I’ve noticed as it became part of our culture that players have learned to keep an eye on the clock and there is more urgency from them as we change from one thing to another.  I build in some time so that a drill/ skill/ scrimmage/ or concept ends at say 45 (45 minutes left in practice) and the next thing will begin at 44.  This usually helps keep things pretty close to on time. Things get smoother as the season goes because they are familiar with what we are doing and all I have to say is the name of the drill and they jump right into it. Sometimes things take longer than planned and you just adjust for the next time you do it in practice.  Most competitive parts of practice that have a time limit just go off of the running clock.  Occasionally I will wipe the clock and put a specific amount of time for situational concepts like late game situations.  I just also have to build in that time.  If I had a consistent manager or other staff members I might look at other options. This has worked well for me.  If you try it let me know how it goes.