I can repeat what I was taught on scanning (by my first instructor, who was an ex-fighter pilot) and has worked well ever since, except when I get sidetracked and forget to do it regularly.
The big issues are (1) for the purpose, peripheral vision is generally much better than direct (ever watch a good martial artist training? - they stare straight into the opponents face, never look at arms and legs, and that's not just to spook them), and (2) anything on a conflicting course will get bigger in your field of view, but won't move much - which makes it all much more difficult (but impossible if your eyes aren't still, at-least for a moment).
So, based upon that the approach I've been using for years is...
- Straight ahead (on horizon and/or instruments as applicable)
- Head quickly left to 9 O'clock level, pause, absorb the view
- Head forward and up to 12 O'clock high, same again
- Head right and down to 3 O'clock level, same again.
(If the aircraft permits, then a brief look behind).
- Head back to 12 O'clock level, same again.
- Resume normal flying by attitude.
30 seconds or so later repeat, continue until in IMC or the circuit.
I'm not saying I've never been surprised by another aeroplane, but it's usually when I allow my lookout discipline to slip and pretty rare when I stick to this pattern.
G