Success is a lot mental - have confidence once mechanics mastered. Everyone hears your every word. Must develop bench, hopefully 2 deep at every position. If you have even talent, then you can sub more - play until you give up 2 first downs, etc. Everyone can come out for a couple of plays.
Gives subs great confidence to play in first.
Tell players all must listen some time during the year it will be very important. Use chalkboard - easy to see and understand. Maybe just to rest or give someone else a chance - or to correct a mistake. Can sub to coach player rather than call a time out. Two heads are better than one.