Our first "real" gig was in a room with horrible acoustics. We had played a couple of practice gigs in a large hall in front of friends and family, that went pretty well-got us used to looking at the audience and interacting with them.
We had always practiced with our monitor mix being the same as the house mix. 2 days before the "real gig" the bass player said we need someone to mix the sound. They came and gave us a separate monitor mix-worked great in practice. At the gig, with the poor acoustics, the amps were turned down so low that the guitar on the left could not hear the bass amp on the right (everything was mic'd) The monitor mix was altered to accomodate. All we could hear was a thick bass mess (no defination of notes) and the singers loudest notes. We could not hear anything (that we needed to hear-like harmonies) and thought we sounded like s**t. I guess the house mix was good, because everyone thought we were great, and have been following us around ever since. My advice-practice like you are going to play. If the other band is in charge of the sound mix, practice with their system-on stage if you can, before hand so you can get comfortable with how it is going to sound on your big night. You will be nervous enough without any surprises in your sound on stage. Good luck.