Yeah..... I used to be AWFUL at sightreading. I mean, TERRIBLE!
Nowadays I'm really good. This is why:
1) I chose quite challenging things to sight-read (Bach fugues, etc.), or just playing slowly through chopin etudes and things.
2) I did it VERY often (half an hour to an hour per day)
3) I used a grey overlay on top of the music (eliminates the glare).
4) I tried sight-reading things with a lot of sharps in. (or flats)
5) I played through something.. and then covered up the music. Then I tried to play as much as possible by memory. This helps, because it means that when you are sight-reading it in the first place, you are constantly looking ahead and working REALLY hard to take everything in.