Enjoyment!  When we enjoy doing something we do it with enthusiasm. We have hobbies, and pastimes which we do because we enjoy them. The same should be said regarding learning or teaching  English. Enjoyment is the key.
This can be difficult sometimes, as there is a lot of repetition and things to remember and practice. But this should not deter the students from having fun and learning at the same time.
In the classroom, student interaction is vital. Getting them to learn from each other, not only English, but also about each other. This is an essential aid to practicing the language. It helps students grow in confidence by learning and practicing with their peers.
In a one-to-one environment, whether that be online or in the classroom, the teacher has to place all of their focus on a single student, which makes the task more difficult but no less enjoyable. I always to try to focus on what the student likes to do and their strengths (such as reading or speaking…). When a student feels comfortable using their strengths, they are more open to practicing the aspects in which they are weaker.
Finally.  There are 2 things any teacher must have. 1st is knowledge of your subject. Students look to us as all seeing, and all knowing. Secondly you must have a passion for teaching. If you don’t have these 2 things, you will not be a successful teacher.