Make it comprehensible! *
The term comprehensible input was first used more than 20 years ago by Professor Stephen Krashen, a researcher into language acquisition. He postulated that a sufficient condition of learning a language is to receive input that is appropriate to the current level of language competence. In the case of a young child learning its own language, this will be predominantly spoken input from parents and siblings. In the case of second language learners, this input can come from a variety of sources and can be both spoken and written. By appropriate to the current level of language competence Krashen means that the input should be pitched so that it slightly stretches the learner - being neither too easy nor too difficult. (Krashen calls this level i + 1.)
If the mainstream teacher can shape the input that each of the ESL learners receive at this i + 1 level - by modulation of the written and spoken language to which students are exposed, through appropriate classroom organization and careful choice of activities and tasks - then she is creating the most favourable conditions for her students, not only to learn English but also to learn the subject content as well. (This is on the assumption that the cognitive challenge of the activities is also at i + 1 level - or in what Vygotsky called the zone of proximal development. Krashen has recently stressed the need for the tasks to be interesting and relevant as well as comprehensible.)
Much of the advice elsewhere on this teachers' site is focused on what the mainstream teacher should know and do in order to achieve this goal of task comprehensibility, interest and relevance.
And finally, another thought which I believe is crucial in ensuring that ESL students can derive the maximum benefit from each class they attend:
* All teachers are language teachers! *
Krashen, S. & Terrell, T. (1983) The Natural Approach - Language Acquisition in the Classroom Oxford: Pergammon
Vygotsky, L. (1962) Thought and Language Cambridge: MIT Press