Aim for the engine. The best way to shoot down a plane is not to tail it unless its aware of you being there then its just a case of manouvering and waiting for the person your tailing to show you his vulnerables

. If thats the case I often find people just circle around and around so climbing above them cirlcling often works and then they get a false sense of secuirty as they thing there not being tailed any more so they go on about there business. At that point you can dive on them and open up at your point of choosing. Usually best is when they've found a tank to bomb and are concentrating on that.
Another tactic is to get above it/behind it and wait for it to turn or climb and fire infront of the plane when it makes its turn (a climb makes them much easier to hit because they lose speed) and wait for the plane to move into the bullets and they will rake the engine.
Have a practise trying that, its all about you choosing when you want to fight and giving the other person no chance to react.