Often one of the most difficult and daunting aspects of a class period is actually starting the class period. Here are a few tips and tricks that I have found to be working for me lately. If you have anything to add, please leave a comment!
1. When students come into the classroom have them all sit down before allowing them to get their instruments. This gets them into the proper mindset for class, and gives you time to take attendance, give announcements, collect papers etc. without having to worry about them making a lot of noise.
2. Have a signal for silence and attention, and maybe even a signal for allowing them to practice while you do something (fix an instrument, find some music, etc.). Preferably something that they can participate in, like a raised hand.
3. No matter how much time it may waste, wait until all are following directions to proceed, or if they fail to follow directions make them do it again. This applies to ending a piece without talking/playing, getting instruments out quietly, entering the classroom properly, etc.