Sciolistes is spot on. Humans, by nature, are rather poor monitors.
Techniques to improve and enhance the inherently boring and monotonous task of monitoring can indeed be taught..
I agree with the first bit - not quite so sure about the second. Improve, yes but really how effective can you become?
There’s monitoring and monitoring. On takeoff or landing, to apply extra diligence for a short while is not too fatiguing. Same during non-normal events. To keep up a sustained active level of monitoring for 5-10hrs plus is much more difficult, especially if the aeroplane is reliable to the point that things rarely go off-piste. That’s one of the reasons why we have EICAS/ECAM, along with other reasons like detailed technical information is not available any more in the manuals.
There’s Human Performance to consider: I wouldn’t say we were poor static monitors, we are
appalling at it! Most of our senses work best when there is changing input - when nothing happens, eventually we start to see/hear/feel, etc. things that may not even be there.
I do a fair amount of LH and ULH flying and I find it impossible to sit there for very long just looking at the aeroplane, especially on a dark night in the middle of nowhere with the other guy on controlled rest. I have to do something that keeps my brain working otherwise I just zone out and start getting micro sleeps.