Training training training practice practice practice: the only defence against the Startle Effect. You have to be so familiar (developed muscle-memory) with doing "similar" things that when the unexpected occurs, you'll have enough spare brain space to think outside the square and realise in time that something isn't right.
We can't train for every specific eventuality, but we sure can do much more training in the groups of events that could cause us grief, GAs being one of them. IMO it is not acceptable to merely have one go at an exercise and say OK, you're good with that or "there's a procedure for that in the FCOM, therefore box ticked". It has to be more than that. Currently, it's not.