My advice would be to practise reading as often as you can. Certain rhythms and note groupings crop up quite regularly, so once you learn to spot them, then it's all stuff that you've played before, so there aren't really any surprises.
If it's a 'sight reading' gig, then I'd echo what's been said on here already; take a couple of minutes to read through the score to see if there are any complicated rhythms or fast runs that need a bit of prep and play them through in your head (visualise where your hands will be on the fingerboard) to mentally prepare yourself for playing them.
Other than that, just relax and have fun with it.