The most important thing is to stay fast. A slow target is a dead target.
Second, patience. If you screw up a pass, don't get impatient and fall into his six hoping you'll kill him before he gets you. Take your time, set up your shots, and make them count.
Third, angles. Don't fly straight at him except when you need to fire(If you can be turning during that, even better) The point is that, from his view as a gunner, you want him to have to chase you around the sky, not have a nice head on gunshot of you.
Fourth, guns. Don't even attempt to kill a buff with .50's unless you have a lot of patience. It can be done, but is mostly just a waste of ammo.
Remember: Virtually all buff's kills are people crawling up thier six. You are an easy target on thier six, so don't be there. The gun passes are everything, you need to present the worst possible target to the gunner.