Read your instructors mind. It's as simple as that. All you have to do is anticipate where he/she wants you to be and be there.
Seriously, the lead instructor should be briefing you and any other instructors, AI's, DM's and candidates before briefing the students. Instructors should layout the plan and assignments to each person and possibly do a walk thru on dry land. Like Rob said, some instructors like assistants behind studenst, others in front. I do both based on class size, # of assistants, conditions. In low vis, I generally like my assistants in front of students in order that they can watch for eye/hand movement to anticipate a panic before it happens.
Once the students arrive, you need to pay attention to the briefing(s) as much or more than the students. Study the delivery and points that are mentioned. When I was DM'ing, I would position myself behind the students during the briefing, kind of mimicing the position I would take up in water. If the instructor moves around while talking, you need to move to position yourself near the students furthest from the instructor.
Different situations (locations, pool, lake, ocean, water conditions, current, visibility, # of students) and different instructors will require different positions for their assistants.
You did not specify an agency, but speaking for PADI, it is incumbent on the instructor to maintain control of not only the students but also positioning of assistants. Each time the instructor completes a skill, the instructor should check that each student AND assistant is "ok", then reposition any assistant(s) as necessary.
If the instructor starts skills on the left end of the line/arc of students, the assistant should be on the right end. As the instructor completes skills with each student and moves to the right, the instructor should gradually move the assistant and eventually have the assistant swim BEHIND the instructor and take up position on the left end of the student line. Don't cut between the instructor and students.
As an assistant you need to have eyes in the back of his/her head. Scanning students for breathing rates, nervous looks, continual movement, fidgety hands, continually touching/holding power inflator, getting distracted by the pretty fishies, PLUS watching what the instructor is doing and where he/she is going.
Don't hesitate to request a staff only pre-dive briefing if you're not having one. Ask where he/she prefers you to be, how far from students (usually no more than arms length in case you need to get hold of someone). Get involved with the students as tehy gear up, familiarize yourself with not just their gear and setup, but also them. The more at ease you make them feel, the easier your experience will be.
I hope this helps and makes sense.
EAP - K.I.S.S. - Keep It Simple Sam. It really just needs to be complete and concise. Just make sure you have your bases covered and if you are delegating resposibilities, make sure everyone is aware of what their roles are.
