Well the Mustang has enough performance that you can dictate the fight against all but a few planes. So if you are defensive then you need to rethink tactics or increase situtational awareness.
If they are shooting at you and you have some speed to get away, use your good roll rate. Roll and pull slightly, roll the other way and pull, etc.
But your best bet against a turn fighter is to put your nose down. Make one sharp maneuver and dive away. If you try to out turn them and go offensive right away you will die.
If you are out of altitude and options, or you are fighting a faster low level plane (Typhoon, Tempest, La-7, Fw 190D-9) then you have to dance. See the discussion of scissors and rolling scissors in this forum.
If you get really desperate and end up in a turning fight, remember to use your flaps. 1 or 2 notches (10 or 20 degrees) will give you a decent turning circle. But you are still not going to out turn a Spit or La-7 unless you are very light on fuel.