Well, I'm far from being an expert flier but thought I would offer my experiences on the "focus" subject.
I got back to c/l flying about four years ago. Big problem with dizziness. I have some blood pressure issues and take medication for that so perhaps that contributed to getting dizzy easily. Gradually, it became less of a problem. I discovered that if I would look away from the plane and focus on a stationary object (tree, car, flag, etc.) for just a second then return to the plane, it helped greatly. I don't have much dizziness now. I think, obviously, it wouldn't be so good to direct your attention -from- the plane while doing maneuvers so I only looked away during level flight. That's contrary to the afore mentioned posts and I understand the need to know where the plane is in relation to the ground especially during maneuvers. Apparently, I do check the background while doing loops and such otherwise I would be buying a new plane weekly. I'm just not aware of doing that (checking the background) when I'm flying.
Again, I'm only offering my experience with what has helped me and not at all suggesting my technique is best. I'm sure with more experience, other methods of viewing the plane during flight will become apparent.
Joe