When your in a turn-fight in a P51B, consider the fight already lost.
Keep your speed over 250mph, and do not turn more than 45 degrees for a firing solution.
If you do have to get low and slow, dont turn so hard that you hear the stall buzzer. The 51 doesnt accelerate very well, so don't blow your speed when enemy aircrafts are nearby.
Heres the planes that the 51B can effectively turn fight:
190A8
190D9
P47 series
P51D
C47
109g
Mosquito
F4U-1,1D,1C