Stay Interesting. The same techniques are taught over and over in class. Long time students have seen the technique taught many times before. Keep techniques interesting by approaching them from various angles and perspectives. Sometimes, instead of starting with a technique demonstration and repetition training on the technique, you may start with seemingly unrelated exercises that lead up to the technique. A back door approach that keeps thing interesting
Encourage special events, and delegate responsibility for them so all students feel more responsibility for the class. Tournaments, camps, seminars, holiday parties, bake sales, and outings are all great ways to stimulate students and break the routine of class.
There is Always the Basics. When stumped for something to teach, there is always the basics: the primary stances, blocks, kicks, and punches. Everyone needs to train on the basics. The beginning students need to learn to do them correctly and the senior students need to remember how to do them correctly.
Keep it Short. Provide adequate explanations. Provide examples from your experiences and toss out bits of Taekwondo knowledge, but keep it short and simple. Too much talk may kill the mood. Class is a learning environment, but is a basically a physical environment. Do not take too much of a break in the physical action.