It's more than one, but here's a copy/paste of a few items I wrote for an ACM site awhile back. They're edited for AH.
1. Ignore what the majority is doing/saying/telling you to do.
Simply put, the majority of players are mediocre at best and terrible at worst. Implementing the collective's advice will simply act to ensure you turn out as mediocre as well. Keep in mind the majority means the majority - your acquaintances are not the exception.
2. Unlearn everything you think you know about dogfighting, and relearn it from the source.
Robert Shaw's Fighter Combat Tactics and Maneuvering is still the textbook reference for dogfighting today because it works. Robert Shaw took the dogfighting and broke it down into it's most basic, fundamental components, and then created modular blocks a pilot could draw on in combat to build tactics on the fly. Contrast that to the way the majority of players learn to play air combat games (this one included), and you begin to realize why rule #1 (above) is true.
3. Being able to fight 1v1 is the most important skill to master.
Every single engagement, whether against one or against ten, can be broken down into a series of 1v1 tactics. If you cannot master the 1v1, you will permanently be at a disadvantage against competent opponents. Master the 1v1 and everything else will follow.
4. Always assume your opponent is perfect.
You should always assume your opponent will not make a mistake. Anything less is presumptuous vanity. Always assuming your competition will fly perfectly quickly narrows your ACM choices and forces you to choose appropriate tactics.
5. Understand your limitations
Contrary to popular belief, there are some fights you simply cannot win if your opponent flies correctly. Learn to identify these scenarios and learn how to mitigate your opponent's advantage during these fights. Doing this will allow you to have realistic expectations, instead of fantasizing the popular opinion that you'll be good enough to win these fights "one day" (see rule #1).
6. Never. Ever. Turn.
Fighter Combat Tactics and Maneuvering, Chapter 4. Those who scoff at this will be doomed to lose to it. Those who understand it comprehend what it actually means.