Yeah, Bach chorales all the way. I'm very intrigued by Bernhard's list and want to check that out. I'm actually the best sightreader I've ever met (NOT the best pianist, to be sure), and the following are what I've done to get more comfortable reading music. I know that people who don't read well have differing problems, so I think it's helpful to try to nail down exactly what's hard for you. Even if it ends up being all the following elements, that's still useful to know.
Many people blow off rhythm until they know the notes, but the rhythm is part of the note. Since rhythmic combinations are pretty consistent within genres, you could go through, say, the Beethoven slow movements as someone suggested (subdivision upon subdivision!) and just figure out the hardest looking rhythms. You would quickly see that they're very similar to one another. I would be very very precise about this, because once you crack that code, everything will look easier right away.
Others who don't read well just don't read BASS clef well. And they sightread and memorize the right hand first, which reinforces reading top-to-bottom, instead of bass-up or, as I do, just a big chunk all at once. If you tend to pay the most attention to the right hand, you might try reading the bass chords in the chorales WITHOUT STOPPING and a ton of other left hand parts for two weeks. Then try to start seeing the shapes of chords, as if it were a drawing, instead of the names of each note. I never think note names while I'm playing, even as a kid I felt it took too much time.
When I was younger, I decided one summer that I was sick of looking down at my hands all the time. I felt it was slowing down my reading and thought if I could only keep looking at the music, I'd read a lot better. And it was true. I spent that summer playing rags with stride left hand, or just making up my own stride patterns. When I learned how to jump around accurately without looking in C major, I started working in all 12 keys. It wasn't tedious at all, it was fascinating. I've never had to work on that again, it went so deep into my memory. If you haven't done this before, you have to be brave. It takes courage to fly through the air blindly knowing you'll land in strange places for a while. But even when it's wrong, it's helpful, because you'll see tendencies right away that will tell you something.
One more quick thing, sorry this is so long, but I think sightreading well is such a pleasure and asset, I hope some of this helps a little... how's your transposition? If you play some really easy things (not your normal repertoire) IN EVERY KEY, you start to experience harmonic relationships in a deeper way, and then you can read in "difficult" keys without anxiety. For me, there are no difficult keys because music works the same way in every key. I guess I've said enough for now! Best of luck.
Topic: Sight Reading Hell..p