Tonight I was invited by two of my plates to go dancing / drinking. One plate I haven't slept with but is probably a sure-fire thing.The other is a definite lay.

I decided to go with the definite lay tonight. What are some things I can say to avoid fallout with the other plate as far as not going with her? I want to fuck her in the future, so what should I say, if anything, to avoid tanking my chances?