I was a Police Driver Trainer for over twenty years. Roadcraft, The Police Drivers Handbook was our Bible along with The Highway Code. These two publications form the basis of all training delivered to the emergency services, the military and by organisations such as the A.I.M. and R.O.S.P.A.
When approaching an exit with countdown markers, 300 yards on a motorway often less on dual-carriageways, the indicator was operated at the first marker post and remained on until the point of no return to the main carriageway.
It was always the rule that it was one indication for one manoeuvre. So, if you were exiting a roundabout for instance with a left indicator on and your plan was to turn left again shortly after the exit, the indicator was cancelled and then operated again for the next manoeuvre.
Also, it was taught to put a turn signal on and let it flash two or three times where possible before applying the brakes. How many times do you see the brake lights come on on the car in front followed by an indication almost as an afterthought. This habit can lead to confusion as the intentions of the driver in front are not immediately clear. Indicating prior to braking also reduces the risk of the indicator being masked by the brake lights, especially where all the lights are installed in the same light unit. I used to have an Audi A3 which had ridiculously small indicators and there are new cars out there with poorly designed indicators.
A more advanced technique applied to the use of indicators was only to give an indication when, it can be of benefit to other road users including, cyclists and pedestrians. This enhances overall observations and awareness of what is going on around you. The habit of simply indicating without determining if it was beneficial or not, was often very hard to break in many drivers.