learn to scan the sky's, head on a swivel. How often is going to depend on how many red icons you can see. Or if your approaching a cluster on the map where you know there is a furball.
If there is nothing on dar, but the con your chasing you can afford to target fixate a bit.
But if your diving into a furball you should be checking your 6 and high 6 every few seconds.
If it helps, try thinking of it in turns of zones.
Zone 1 = Anything that can get his guns on me right now or before I scan again.
Zone 2 = Anything that can get in guns range in the next 10-15 seconds.
Zone 3 =Anything close enough for you to see their icon.
Part of it is built up over time, each scan you notice where planes are at, where they are heading. If they are a current threat, about to become a current threat, or could become a current threat.
The real trick, is when your chasing someone, trying to line up a shot, remembering to do that quick 1 second scan.
So if what your doing now in that situation is Maneuver, estimate lead, then shoot. You need to add a quick scan to that.
So it would be look, maneuver, estimate lead, shoot, and look.