Perhaps it was mentioned above and I just missed it, but there are two things that I think are essential to good sight reading, and they are aiming for significant beats, and internalizing common rhythmic patterns.
When playing, identify the rhythmic phrases and motifs, usually in small units, and identify which notes (beats) are the most dominant. Then when you play, aim for those beats, nail them and then go back and fit the smaller rhythmic units into them, usually preceding them.
Most music you're playing does not really have all that much rhythmic variety to it. You can break a great deal of music down into a lexicon of rhythmic patterns that you have learned. Then, when you play, keep thinking ahead of yourself in the music while identifying (on the fly) various rhythmic patterns.
- - in other words, if you have two sixteenth-note runs into an eighth-quarter-eighth, don't read, "sixteenth-note runs - - eighth-note - - quarter-note - - eighth-note singly", you read both of the runs as one unit and you read the eighth/quarter/eighth as another single unit.