Look at the music first and get a feeling for the rhythm, and tempo.
Getting all the advance mental preparation you can, even reading 'at sight', is not breaking any rules. The clearer idea you have of the music before you start playing, the better the results will be. Try to 'hear' the music as far into the piece as you can.
Also: train your eye to jump to bass and treble notes on downbeats and other strong beats. If you have to skip, or leave things out, these will anchor you and keep you on track - keep the rhythm and, if necessary, jump to the next downbeat or strong playing the outer voices.
This keeps you on track particularly if you're reading with other musicians.
With the outer voices, you should also get an instant grasp of the harmony - that will help fill in inner parts, even if you don't play every single note. You see a c minor triad, you can fill it in harmonically - etc.
Finally, try to train your eye to look ahead of where you're playing. This takes skill, but is the biggest secret of expert sight-readers.
Those tips should really help out.
Also: just read a lot music you like to explore, not too hard - give yourself pleasure and enjoyment, and the reading will come along on its own.
Then take a try at my piano etudes ...
peace, CD
Topic: Sight Reading