Best tactic for getting better with the P-51 is to find someone better in it who flies like you do to wing with, feed off their table scraps and figure out what they do that would work for you. Everyone has different strengths and weaknesses, what works for someone else could be incompatible with your flying style. To this end, I've been slowly leeching off Fencers P-51 Mojo for about a year now.
Also you have to figure out which setup is better for you, everyone is different there too. I now fly the B almost exclusively, even though I started out in the D and took forever to realise I liked the razorback better. Now the B is probably my favorite plane in AH.
Personally I'm a lousy dogfighter but decent shot, so I usually make fast atacks and try to as much damage as I can as quickly as possible (unfortunately, that means I also get close enough to hit falling planes a lot...) and then look for another target. I get a lot of kills clearing tails and tag-teaming larger flights. I wouldn't really call it BnZ, since I'm usually right in the middle of the furball, but I stay fast, avoid turnfights when I can, and try to take them down as quickly and efficiently as possible.
As Duane Beeson so quaintly put it- "...sneak up close behind and hit 'em in the back of the head with a baseball bat."
