One of the parts that I've improved a lot in the last year.
I think I finally figured them out and is a fairly easy step... And the reason why I always do the eyes last, is because once they're done, the subject looks "finished" and if I add the eyes too soon, is more likely that I'd make a mistake and I won't be able to see it until the whole face is done and something seems off; and then I'll spend another couple of days just trying to fix it.
So by leaving the eyes blank, I can evaluate if the likeness is right in real time, that's the thing, I can only focus in one thing at the time.