Minimal body movement
Excess upper body movement is very bad for your balance. Even those people who are well stretched sometimes throw their upper body around especially whilst kicking. It’s important that when practising your back is as straight as possible.
A prime example of this is when students perform side kick. They kick forward yet lean right back, making it easy for students to become overpowered by their oppoenents. This particular kick would be a lot stronger and more powerful if students pushed their hips forward and kept their back straight.