What general principles do you apply to the teaching of ESP, whatever the field?

The 2010 BESIG conference is fast approaching, and I've been busy preparing the talk that I'll be giving alongside my good friend and long-time colleague Mark Ibbotson. Mark's a fantastic Business English and ESP author, responsible for the Business Start-up series (CUP) as well as Cambridge English for Engineering and Professional English in Use Engineering (both also CUP).

As Mark and I have both written books in the Cambridge English for … series – him on Engineering and me on Marketing – we thought it would be interesting to look at what, if any, commonalities existed between the teaching of English for Engineering and English for Marketing. That lead to the following talk title: Different fields, common ground: From marketing to engineering, some thoughts on the ground rules for successful ESP teaching across all fields.

While researching the talk, I've been giving a lot of though to what, if anything, works as an overarching methodology for the teaching of ESP. I want to avoid being too prescriptive, so I've come up with a list of things that are important to me, as a teacher of ESP and a writer and editor of ESP materials. This list is very much a work in progress (and it's highly subjective), but at the moment it looks like this:

1. Engage your students not only as language learners but also as professionals.
2. Choose your focus correctly – i.e. vocabulary (what type?), skills / functional language, grammar (general grammar or task-specific grammar), etc.
3. Think about who your students need to communicate with (their "web of relationships") and the channels in which that communication takes place (written, spoken, etc.). Use that as a basis for the texts and contexts you're going to focus on in your course.
4. Teach language not theory.

So that's what I'm working from at the moment, and I'll be fleshing those points out between now and the conference. But what would you add to that list? What principles do you think could be applied to the teaching of ESP, regardless of the field?