Whatever your texbook or reading material, chances are some of or all of your students will tire of it sooner or later. Usually sooner. The textbooks are there as a jumping-off point but not a sole resource, and it's up to you to turn the material -- grammar points, conversation starters, dialogues, dictations, vocabulary exercises -- into something approaching fun. Always plan ahead a few classes, looking online for games related to the grammar point or for supplementary materials appropriate for the age level and interests of your students, which could be anything from a simple crossword puzzle to a magazine article about drought in Africa.
The worst possible start to a class is to say "OK, open your books." These people, your students, see you every day and look to you as a resource for language. They can read the books on their own. In most cases, students want to practice conversational English, and the best way to do that is to start each class -- even if it isn't a conversation class -- with a bit of talking. Ask easy questions, like what students did the night before or to tell you their plans for the weekend. Have questions prepared for each day and begin with a few minutes of easy talking to get students out of their shells.
Don't be afraid to plan times for movies and videos into your class and think beyond the three-minute YouTube video. Try to gauge the interests of your students and plan a movie day around that. Watching a movie with an ESL class regularly is an effective way to reinforce language. Pause the movie frequently to ask questions about the content or to explain difficult concepts when students get that glazed-over look in their eyes.
When teaching ESL to children, be patient. Kids won't synthesize new grammar points or pronunciation as quickly as adult learners, so correct them gently when they make mistakes and wait for improvement. If a student is repeatedly mispronouncing the word "ice cream," don't just say the word correctly and move on; use the word in a conversation, saying it correctly, and let the student hear it. With adult learners you can be more direct, since these students usually want to be corrected intensely to improve quickly. Know the personalities in your class and make corrections that won't embarrass anyone.