Take a look in outside view - you can see every single spent cartridge flying away!!
My two most common mistakes in IL-2 are:
1) Attacking bombers from the rear. The rear gunnners are pretty accurate (and seem invulnerable!).
2) Attacking AAA. Can be done, but I don't know the trick!
Other hints:
1) Watch the torque on some of the more powerful planes - and keep trimming.
2) Watch your engine temperature. Monitor your revs and use prop/mixture controls.
3) Some aircraft have a turbocharger (as well as WEP).
4) Some aircraft can be decidedly "twitchy" - particularly the Fw190.
2) Also your radiator can help with engine temps (good idea to open it in a dive, let some fresh air in
3) Yep we have to change our supercharger at different heights for some aircraft.
4) They sure can, the P51 is known for spinning out, takes a smooth hand to fly some planes. Also setting your control inputs to smoother settings (can do this from the main menu ingame) tends to help twitchyness.