I'm not much of a pilot, period, but here are my general pointers:
1) Practice your deflection shooting. Especially with 0.50 caliber-armed planes, you need to be able to hit with a higher percentage of your shots. You can't hope for a lucky hit with a 20mm cannon to take off your opponent's wing. Instead, you need to be able to aim a "squirt" of bullets, at least 2-3 seconds long at a vital part of your opponent's plane, like fuel tanks or the engine. You really need to aim at "center of mass" (i.e., a cross shaped area centered on your opponent's cockpit, taking in his wing roots, engine and fuel tanks).
Edit: Good shooting also means only using your guns at the range where they converge, and perhaps setting your convergence point to a shorter distance than default - say 150-300 m. This is especially important when flying a plane with wing-mounted guns (like most of the U.S. fighters).
2) Keep your speed up. Especially avoid hard climbs and turns which bleed off energy fast and can leave you as a predictable, slow target. If you get into a low-speed turn fight with a better maneuvering plane - like a Spitfire or, especially any of the better early war dogfighters (e.g., A6M2, Ki-43, B-239) you're throwing away all of your advantages. Even if you're not against a better turn-fighter, at least speed means that your opponent has to pull more lead (i.e., aim farther ahead) in order to hit you.
3) For the Mustang, fly with 50% fuel or less. While any plane is going to fly better if there's less weight in it, the Mustang is modeled so that the tank behind the pilot is emptied first. Realistically, this changes CG (center of gravity) forward, helping to make the plane more maneuverable. Unfortunately, realistic fuel management (i.e., which tank you empty first) isn't modeled in IL2, but I think that CG changes from reduced fuel are. Anyhow, the Mustang has outstanding range, so you don't have to worry about running out of fuel even if you start with less than a full tank of gas.
4) Bring some friends. Seriously. One of the reasons that the Mustang got its reputation is because there were more American pilots in the sky at any one time than German or Japanese pilots. Even if your wingmen suck, they still act as bullet magnets and distractions for the enemy. Your AI buddies may die like rats, but at least they die nobly by setting you up to make the kill. Ideally, you'll actually be able to control your wingmen, so they do something more useful than die. The "cover me", "rejoin" and "attack my target" commands are especially useful.
Another way to "bring friends" is to make other aircraft visible in the map view, although realistically you shouldn't zoom in too much. This represents other friendly planes in the area calling in reports of enemy activity and location. Americans had some the best radio and radar equipment for the period, and they made extensive use of it.
5) Fly 4.11 against Veteran or worse AI opponents. The new patch is a revolution in AI behavior. Their engines overheat, they can stall, they can be surprised, they can't see through clouds, darkness or terrain. Even better, they are actually challenging opponents, even at the lower levels of AI skill, so they don't teach you bad habits like the old AI did. At Ace level, a one-on-one fight is actually a workout for a moderately competent human pilot, and 1-vs.-many encounters against Ace AI usually result in a dead player.