1) Keep it fun.
2) Keep a good level of discipline.
3) Mix it up in your sessions so that kids stay interested.
4) Teach sportmanship and fairplay as these will not only help your students in tennis, but also in life.
5) Comparing kids with eachother is one of the worst things you can do for their self confidence.
6) Keep things time based as kids normally have a fairly short attention span.
7) Use appropriate equipment for your drills.
8) Try to incorporate as many games as possible in their daily routine. Kids love games and you can use this to help them learn faster.
9) Provide positive feedback and encouragement whenever possible.
10) No long speaches or lectures.