I don't shoot at farther than 600 w/50 cals, less with German, Russian, or Japanese cannon.
Main thing is to be very smooth on the controls and focus on the target. Fire short bursts, observe where they go an and correct your aim smoothly. Don't jerk the plane around or the nose bounce will be hard to control. Straight six shots tend to be the most difficult to get killing shots. 90 degrees off, if you can hit, tends to do more damage.
I'd strongly recommend flying just one plane, or at least one kind of gun to really get your lead figured out. By that I mean, if you fly American planes, stick to the 50's so that you can get used to the trajectory of that gun. If you fly German, stay with one main gun, either the 20mm or taters. In planes that have multiple calibers (P-39, Spits, etc.), figure out which is the main guns you want to optimize for and work on hitting with those. These are usually the ones with the most hitting power.