One of the best lessons I have learned recently is "stay cool". Keep radiators open prior to combat. Keep RPMs lower and prop pitch back a bit. Don't leave WEP enabled in combat by default. Throttle back in dives, reduce prop pitch. Open the radiator whenever you can in combat.
If the RPM's and prop pitch are cut a bit and you need to pour on power, the plane will jump when you do. Being "cool" prior to combat leaves you a bit more room for the temperature increases in combat. If you don't need WEP to catch someone or get away, don't use it. That's a bit of reserve power to call on when you need it.
Probably the hardest thing to learn is not bleeding off all of your speed in a turn (when you don't have to). Speed is life after all. Much of the time a wider turn at higher speed is preferable to a tight turn that leaves you just above stall (depends on the situation obviously). As I get more experience in the sim, I find myself being more "smooth" with the controls to preserve speed.
Trim, trim, trim. I am obsessed with the slip ball

. But it helps in flying efficiently and maintaining or gaining speed. When cruising or trying to catch up to an enemy, I find that climbing or diving with elevator trim is a big key in keeping as much speed as possible. Re-trimming on approach also makes landing easier...and safer

.
Splitter