[Griefbringer]

What Mobhack said. Spotter planes are slow and not very robust, and the spotting runs take quite a lot of time - thus allowing any anti-aircraft weapons close enough to take a lot of shots at them. So have them flying over the areas that you control, and spot from afar.

If you want to run spotting runs closer to the enemy controlled areas, then it is good idea to use your artillery assets to damage (or at least suppress) the enemy AA assets. If you have enough artillery, you may just try to pound likely positions in the rear. Alternatively, you may try to draw out AA fire with aircraft in order to find the approximate locations of the AA units - fast and armoured ground attack aircraft are more likely to survive such missions than spotter aircraft.

If you are fighting an opponent with lots of vehicle-mounted AAMGs, such as the US army, then it can be very difficult to take out all of the AA weapons.