Whatever you do, DO NOT listen to anybody who tells you to try to ALWAYS have the altitude advantage over every enemy plane in the area. That will turn you into an "altmonkey"...it WILL NOT improve you as a virtual pilot. Plus you'll just be wasting a lot of time climbing only to find you blow all that excess ALT 5 seconds after you enter combat.
Get off the deck, up to about the altitude you see everyone else flying at. That's a good start.
Here are 3 rules of air combat in a crowded environment:
1. ALWAYS check your rear and rear-up view at MINIMUM of every 8 seconds in a fight, better still every 5 seconds. Check the clipboard DAR BAR at minimum once every 30 seconds. Check your tail as you prepare to fire and immediately after you fire; many pilots will attempt to attack you when your attention is accupied as you line up on someone. Keep tabs on the odds in the area; you'd be surprised at how quickly a 10 vs 10 fight can degenerate into 6 or 7 enemies all piling up on you. If things seem to be going sour, it's your choice to either fall back and regroup or attack and go down fighting.
2. DO NOT ever get "fixated" on a single enemy in a multi-plane engagement....do this and you WILL die in short order. Feel free to get slow and "mix it up" with someone, but always watch out for somebody trying to pick you off, because trust me--it'll happen. The converse of this is also true--if you see an enemy lining up on a friendly, blow him to hell while he's not watching his tail.
3. Don't choose your targets based on what model they are. When there's a Spit and a P-40 co-alt, most pilots will attack the Spit and ignore the P-40, considering the Spit to be more of a threat.....then that P-40 will turn around and blast you. Pick your targets based on how much of an immediate threat they are and on whether you can kill them or not--NEVER make the mistake of discounting a plane just because it's a "bad" model....very often the "bad" planes are flown by the best pilots and are actually a greater threat than the common models. Fact is, if an airplane is in a position to shoot at you, then it can kill you just as dead whether it's an N1K or a 202.
I hope these pointers are of use.
J_A_B