Sightreading is a matter of forcing yourself to learn chordy pieces (start easy) and just playing lots of stuff you don't know..
btw, sight-transposition on organ is killer... *shudders* thankfully, haven't had to do that for ages.
Anyway, back to the point... if you want to learn to sight-read, do lots of it. What helped me (I think) is looking at the chords, and playing the closest thing you can play to it... if you try and learn your pieces by sight-reading it'll help after a while!