A very effective method of working on sight reading is to take a book (graded as suggested earlier), that is one or two levels below you. Just sit and play through the book one day.
I moved to Vancouver for school after having finished my undergrad in piano, and over the holidays back home, I didn't have any of my books. So I've been playing through the grades 3, 4, and 9 RCM haha! My mom loves it because all the pieces sound so innocent in the earlier books, but it's just something fun to do on the side.
When I was practicing a lot more seriously (leading up to audition for university and during), I'd often pick grade 9/10 pieces and give myself about 10 minutes to work through the kinks and try getting it. So a piece like Claire de Lune, Chopin Op 9 #1 Nocturne, Grieg Notturno, Ravel Pavane, etc.
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On the other hand... while I was in high school I was OBSESSED with stuff like Scarbo, Rach 3, Prokofiev Toccata, etc. etc. So I feel like my internalizing the reading aspect of really difficult music came first, and then later I worked on polishing my sound.
For me, I found two points in sight reading... getting the notes down (which is what I was fixated on in high school. Like saying "WOO! I played a page of Scarbo after 3.5 hours!!!!") and then making some music out of a piece (today, I sat and played la Fille au Cheveux de Lin by Debussy, and after playing it I just kind of sat back, took a deep breath, and said "holy sh*t, that was amazing")...
The Scarbo example was me earlier, trying to grind through difficult passage after difficult passage, rendering slightly-difficult passages in other pieces into something that was not challenging to play through. The musical quality was severely lacking though...
The Debussy example was something I developed later... picking a Grade 3 RCM piece or whatever, and possibly trying to play it a semitone higher at sight, or playing it in the parallel minor key, just to see if I can add a musical challenge rather than MOAAARRR NOTESSS. So, playing the Debussy, because the actual "read the put your fingers in place" is no longer an issue, I had time while 'sight reading' it to shape it the way I wanted.
I can't answer threads like these without turning my response into a rant-like post haha.
TLDR: Maybe pick one 'challenge' piece and try learning a page of it in an hour. And then maybe take 10 pieces from an easier collection, and play them as if you had to record them at a studio the next day.