Start with the basics. Numbers, time, colors, days of the week, months and seasons are so basic it is easy to forget to cover them, but beginners will definitely need a lesson in these basic terms. For intermediate students, focus on reading prices and phone numbers, clothing and items in the house.
Create vocabulary lists based on common words that are associated with each other. It is easy for students to forget vocabulary words and it helps if they learn new words in related groups. Choose a topic, write your vocabulary words, then separate those words into smaller groups. For example, if the topic is "Summer" your groups could include weather, beach, vacation and marine life.
Teach basic phrases before grammar. It is important for students to know why they need to say something a certain way in English, but when they are beginning to learn it is easier to remember entire phrases, such as "How are you doing?" For intermediate students, have them break down the phrases, identify parts of speech and re-conjugate in another tense.
Repeat, repeat and repeat again. It is easy to forget how much repetition can help ESL learners retain new language points. Each time you introduce a new vocabulary word, repeat it at least three times. Repeat your example sentence at least three times. Have your students repeat it at least three times. You get the idea.
Use multi-media materials such as songs, TV shows, commercials or movies. Commercials have a short format with strong visuals, simple language and lots of repetition, which makes them particularly useful for ESL learning. TV shows are also useful in the ESL classroom, just make sure you choose programs that are appropriate for the age and cultural background of your students. When showing movies, break the movie into segments and prepare exercises for in between each break.
Avoid idioms and expressions. Idioms such as "let the cat out of the bag" and "something's fishy" will be useful for your students eventually, but are mostly confusing for beginner and intermediate learners. Review your material carefully to verify that it does not use too many expressions; it is almost guaranteed that your students will want an explanation if they hear an idiom.