Mert Bland on developing English training for special purposes (ESP): "I always begin where the students are and work outward in a kind of spiral. Like, have each one describe what he does. Ask questions. Have him teach you what security and facilities maintenance are all about. He should know more than you."
Mert makes a good point here. It's very easy for the teacher to make wrong assumptions of what the students need. You should also ask your students for copies of their incoming and outgoing Emails so you can see what your students communicate about, what kind of language they use and what kind of errors they make. When I start a training I get 10 Emails from each student. I have collected thousands by now and have a pretty good insight into real business writing.
Many teachers think that the students need to expand their vocabulary in their professional field. However, I've found that professional vocabulary is one of the things they learn first while on the job and they are often more in need of improving their grammar to support their vocabulary. This is a big relief for the teacher who may not be conversant in their particularly technical field.
Lower level students will need more functional help, how to describe or plan certain actions; ie: planning the work schedule for security staff. Higher level students will not have much trouble with this but will need help in discussing concepts and ideas; ie: future trends in security and maintenance.
The best way to figure out what they need is, like Mert suggested, talk to them about their work. You could also ask for a tour of their facilities and this will give you lots of ideas of things to talk about with them or you can even help them learn how to do the tour in English.
One final thought is that in some cases it is best to not always fit the training too closely to their job. I was teaching Business English to some students from Caltex and found some Caltex materials and a video clip on the Internet. I formulated some lessons around this Caltex material which I thought would really hit the target of what the students need. I suppose it did but the students hated it. They told me they deal with Caltex stuff all week and they'd like to get their minds off work a bit when they studied English.