It is important to assess the extent of your students' skills and musical knowledge before attempting to put together lesson plans. This is especially important if you are teaching single students or very small classes. If you are teaching someone how to play a specific instrument, ask her to play a little music for you. You may want to come up with an introductory test on the basics to see what your students know. The better you can tailor your lessons to your students, the more they will learn.
When attempting to teach music, it is important to listen to music. If your class size and time constraints allow it, ask each student to bring in a sample of music that he enjoys. It is also a good idea to play well-known classical compositions as well as examples of many different styles, from Gregorian chants to modern jazz. A day or two of listening exercises can help you get a feel for the tastes of your students. You can also use the songs you listen to as basic lessons in teaching your students to find the beat as well as distinguish between major and minor keys and different time signatures.
If you are teaching students to actually play an instrument (as opposed to a music theory or general elementary music class), introduce them to visual cues early. For beginning students, you can use simple cues such as touching your head to start playing and touching your hips to stop playing. For more advanced students, teach the motions that conductors use when leading large symphonies. Also, if you are teaching music reading, make sure that the students understand the dynamic markings, repeat signs and other odd cues that adorn pieces of sheet music.
Instead of focusing on technical matters all the time, take a class period every once in a while to explore the history of music and musical styles. Delve into both classical music and the history of modern music. Again, exactly what musical styles you want to explore will depend on the age of your students and the type of class you are teaching.
Music theory is technical, and as such it is not simple to teach. Many worksheets and workbooks exist that can help teach this difficult material, but you have to remember that students will get tired of constantly doing worksheets. If you are teaching advanced classes, try to break the monotony by analyzing some real music, both classical and contemporary.