The best thing you can do for sight reading is to learn as much notated music as is humanly possible and try to do it as quickly (and cleanly) as possible
this link was posted on here a while ago and I saved it... should give you plenty of material... just set a metronome to a not too fast speed and start reading... try not stopping the first time through and see if you make it. then work some stuff out and play through it again to finish... then move onto the next one. You just gotta keep in your head that everything has to be perfect.
http://www.lucaspickford.com/transbass.htm
Also once you get decent you can try and get into a pit orchestra/band at a local church, school, or community group it's really great sight reading practice... you don't get that kind of pressure anywhere else