When talking to people about this specific problem in class, I usually surprise them with my comment: "stop standing on your heel."
If you are light on your heel but strong on the ball of your foot you have several advantages:
- You are using your calf muscles. They're very strong and confidence inspiring.
- You have less rotational friction than a planted foot. It's easy to rotate on the ball of your foot and maintain balance. It's possible to rotate on your heel but very hard to stay straight up.
- You are less likely to hurt your knee (or to hold back to avoid injury). A planted foot does not want to rotate and will force you to twist your knee or keep your hips back. Either will lead to an off-balance weak kick.
- You are ready to launch another kick from your bottom foot. Kicking from a planted foot is awkward at best.
In terms of exercise to strengthen those lower legs and the transition, I would suggest jumping jacks. Admittedly, this is because almost all classes will have you do a lot of these so it's an easy sell. Jump rope also works but I never seem to find time....
Concentrate on keeping your weight on the balls of your feet rather than stomping down with your whole foot on each jump. Keep those feet close to the ground rather than bouncing up high. I found that this greatly improved my footwork, my dynamic balance as well as my stamina.
By controlling my effort and using my stronger muscles, I went from 60 jacks seeming like a lot to the point where I'll warm up the class with jumping jacks until I get bored (and they've all turned green :-).