Aaaaaannnnd . . . the hits just keep on coming from teaching pro John Carrizosa (did you see what I did there???)
In this video, John suggests using a simple penny to build a good swing habit (or break a bad one) in your beginner students. Balance a penny on the top of the frame of the racquet. Racquet strings should be perpindicular to the court, or facing the net if that is easier for your students to understand. Have them practice swinging slowly and carefully so that the penny stays balanced on the frame. Works on both forehand and backhand sides.
I have some students who have trouble keeping the racquet face neutral as they swing. The ball either goes flying (racquet too open) or dives into the bottom of the net (racquet too closed). I think this comes from them being too wristy, possibly from playing Wii Tennis. Now before you start sending me emails, remember that I love Wii Tennis. This is just about the only knock I have on it. In some cases, I think using the Wii remote in place of a tennis racquet teaches players success with wristiness, which definitely does not transfer to the real game. Racquetball, maybe. Tennis, no.
Now I realize these pennies are not going to stay on anyone's frame very long. But the visualization is worth many times the penny's value. When you see your students starting to open or close that racquet face at the wrong time, after using this activity, you can just say 'penny' and they should be able to auto-correct.
p.s. as in the video, be prepared to cough up the penny - your students will def ask if they can keep it!
UPDATE: I guess it was inevitable. Last time I used this, there was almost a Penny Riot on court. Someone lost their penny and accused others of 'stealing' it. So consider retrieving the pennies after the activity and re-distributing them at the end of class. :(