I was sight reading just now and I noticed that I'm applying accidentals subconsciously. I used to have to mentally keep track of them but now I noticed I played a passage and wasn't even thinking about the accidentals and I played the passage correctly.
I suppose that is why when you ask advanced sight readers how they do it they mostly don't have a precise answer other than "practice". I think if someone asked me just now how I did that I would have no clue. I read the accidentals on the page and my hands figured out where to go and my brain somehow kept track of which ones to apply when. There were even some naturals to cancel them and somehow it came out fine.
So yeah, now I get the people who say it's "practice" and don't offer any more help. If you do a lot of it your brain starts picking up the details subconsciously.