Giving Instructions
Do give verbal and written instructions -- this practice can help all learners, especially ELLs. In addition, it is far easier for a teacher to point to the board in response to the inevitable repeated question, "What are we supposed to do?"
Don't act surprised if students are lost when you haven't clearly written and explained step-by-step directions.
Check for Understanding
Do regularly check that students are understanding the lesson. After an explanation or lesson, a teacher could say, "Please put thumbs up, thumbs down, or sideways to let me know if this is clear, and it's perfectly fine if you don't understand or are unsure -- I just need to know." This last phrase is essential if you want students to respond honestly. Teachers can also have students quickly answer on a Post-It note that they place on their desks. The teacher can then quickly circulate to check responses.
When teachers regularly check for understanding in the classroom, students become increasingly aware of monitoring their own understanding, which serves as a model of good study skills. It also helps ensure that students are learning, thinking, understanding, comprehending, and processing at high levels.
Don't simply ask, "Are there any questions?" This is not an effective way to gauge what all your students are thinking. Waiting until the end of class to see what people write in their learning log is not going to provide timely feedback. Also, don't assume that students are understanding because they are smiling and nodding their heads -- sometimes they are just being polite!
Encourage Development of Home Language
Do encourage students to continue building their literacy skills in their home language, also known as "L1." Research has found that learning to read in the home language promotes reading achievement in the second language as "transfer" occurs. These "transfers" may include phonological awareness, comprehension skills, and background knowledge.
While the research on transfer of L1 skills to L2 cannot be denied, it doesn't mean that we should not encourage the use of English in class and outside of the classroom.
Don't "ban" students from using their native language in the classroom. Forbidding students from using their primary languages does not promote a positive learning environment where students feel safe to take risks and make mistakes. This practice can be harmful to the relationships between teachers and students, especially if teachers act more like language "police" than language "coaches."
This is certainly not a complete guide -- they are just a few of the most basic practices to keep in mind when teaching English-Language Learners (or, for that matter, probably any second language learner). What are more "do's and don'ts" that you would add to the list?