Oz is right on. My mom ( A piano teacher) always had a sight reading pile, I have the same, tons of pieces on my music stand.
There are also some important distinctions:
1. Being able to read music means seeing a piece of music and knowing what it says. I can "read" nearly anything at this point, It only question of being able to play it or not.
2. Sight reading for us requires a combination of reading music and playing the bass. Many people get a "deer in the headlights" thing going and get too caught up in page and forget about the instrument.
3. Once you have read and fingered a pitch move on the the next thing. Practice this slow at first, and it will speed up.
4. Never be afraid to be redundant with music. Say the alphabet to yourself if you need to (up to G at least), it seems to me the part of our brains that understand music are disconected from the part that learned the alphabet as a kid.
Put some time into all three clefs as well common octave transpositions (octave up (or at pitch however you want to think about it) on all three for certain italian solos or 'cello music, octave down on treble comes in handy for playing jazz and other related music. It is kind of a hassle at first, but once you learn them you know them. Get it out of the way.
I also pick up those little pocket full scores of Brahms or Beethoven or whatever and read along while listening to the music. I will often read whatever solo piece I am working in bed or on a plane, thinking of the fingers and sound.
