Having clear behavioral expectations in places the child can see them can help. For instance a sign saying please sit still and listen could be posted behind the place where the teacher often talks. The teacher may then point to the poster as a first reminder to come back on task. Therefore when you have an expectation that a series of instructions will be followed they will need to be presented more than once and in more than one way. They will also need to be presented so that the child can refer back to them as need be.
Eye contact is a good way of bringing a child back to task.
Seat the ADD child near to your desk and make sure that the child is within your line of sight most of the time. This will help the child stay on task. Don't fall into the trap of getting into discussions where the child is acting as a barrack room lawyer. These are not meaningful to the child and they only serve to wear you out. If the child needs stimulation then the child needs to be encouraged to say so in an assertive way.