music theory
I used to be quite poor at sight-reading (well I thought I was anyway) but now I'm pretty good. I think I've got better because of my knowledge of music theory, as well as lots of practise obviously.
Learn about keys, scales, chords, common chord progressions, common patterns etc. A lot of these are best learnt in practise, by playing loads of classical music etc.
I would draw an analogy with reading (as in books). When you read, you don't see a load of letters - you see words. This is because over the years you have learnt to spot patterns. It's the same with music - once you see 'chords' instead of notes, you can sight-read more quickly, and even 'improvise' slightly when you're not quick enough to read the music.
If you're sight-reading for exams, then it's better to do it very slowly and get all the notes right, rather than more quickly and making mistakes. But if you're accompanying someone, the opposite tends to be true. And finally, if you're struggling when accompanying someone, it's better to play less notes (e.g. just play with one hand while you're 'catching up') than to keep going ahead and making more mistakes.