For sightreading - here's my advice:
Take lots of time to mull over the piece.
Get the overall impression - legato/stacatto, dynamics and tempos - imprinted in your mind - you get marks for these - and it's easy once started to forget all about them when the going gets rough.
No matter how awful the piece is, DON'T STOP once you start if at all possible.
Don't be afraid to throw in a few 'odd' notes to tide you over (in the same key) if you're overloaded and there's no time to read them! Lets face it, nothing at all is going to be wrong.
I sympathise very much with you - this was always my weak area and I used to dread it. Luckily its not worth 'that' much, so you can afford to totally mess it up and still do very well.
BEST OF LUCK! and do post us all a note on how it went!