Practise practise & more practise!
Partly depends on what you fly. IE its easier to get hits in MG armed planes, but its easier (IMO) to get kills with cannon.
Next, if you are flying with cannon try setting them 50 longer than the mg's.
This should help get them both into the target, esp at ranges over 250.
Check your fire with the .target 300 command. This brings up a bullseye. (Its always North of you) Lets you see what kind of dispersion your getting, etc.
Range on target is in meters, make it go away or reset with .target 0.
Kind of like shooting Geese, start with crosshairs right on target, pull through in the direction he's traveling as you fire. Longer bursts at first till you start seeing when it is your hitting. But until your hitting almost every burst don't worry about the ammo.