i think it starts with a good foundation when you begin piano lessons. part of the lesson is learning where the notes are. with both reading language and reading music - you have an interpretation of symbols going on. if you try to learn too many too fast - you're always going to expect slowness. but if you take five symbols a week and really learn them well - you know them. you don't have to guess.
guessing isn't the game. knowing is.
this is of course, my own opinion and approach - but i think with very young students - you can use the same 'look ahead' principles, too, as with reading. using a index card to cover up tot he point you want the student to read. for instance, if they play one chord (or even one note in rh and lh together ) then you hold the index card over all the other notes excepting the next chord that is coming. when they can handle looking ahead one beat - then you add two beats. you let them read it first (scanning is always a good idea). then play without STOPPING.
stopping gives you a cue that you haven't read that far. also, there is the element of coordination. some students will always be a little less coordinated than others. but, that doesn't mean that there isn't always room for improvement.
by college - one, imo, should be able to scan an entire page and then play. (not from memory - but from 'reading' the text). of course, when you are literally sightreading and playing - you may only read a line at a time. but, your eyes get used to taking in more notes as you get older.
ps i think that flashcards are invaluable to first and second year students. for older students, sightreading a lot of different pieces that are a level or two below where they currently are. and, joining a choir - and accompanying. my sightreading took off when i was expected to play music from sight (accompaniments).
Topic: The secret to sightreading...