Don’t look at your hands: keep your eyes on the music. If you have to look, glance with your eyes, but don’t move your head.
Learn to look one beat ahead: quickly memorize each beat before you play it so that you can already be looking ahead while you are playing. As you get better, you will be able to recognize notes and chords faster, so you will be able to look farther ahead as you are playing. (This doesn’t mean stop, memorize, go, stop, memorize, go… what it means is that you are constantly playing, but instead of always focusing only on what your fingers need to do RIGHT NOW, you are always looking ahead, having already quickly read and remembered what you are playing right now – that way, you have already seen every note that you play; albeit, you have only just seen those notes a few seconds earlier. Especially when you see that you have a rest coming up, that is a GREAT opportunity to look farther ahead and remember what you have seen.) It doesn’t give you a lot of preparation time, as you really are only seeing for the first time what you will be playing in a second or five seconds or ten seconds, but it certainly helps.
Practice scales and arpeggios and chords without looking at your hands, so that when you are sight reading, you don’t have to look away from the score to play such passages. Practice scales and arpeggios and chords so that you will recognize the patterns in music when you seen them, and so that you won’t have to worry about fingering.
Learn to recognize patterns: see notes as parts of chords, not just as single notes.
Remember what you have already played. Remember the key signature and time signature. Remember accidentals. Remember recurring patterns. Remember harmonic progressions (if you can recognize the progressions as you are playing: we actually have to do this in my class. As we are all sight-reading together, say, a Beethoven Sonata, one of the students is assigned to shout out the chord progressions as we (and that student) sight-read the music.)