Pre-Show: If it's your first time working together, exchange information about what's off limits. "I'm PMS-ing and my breasts are sore, so be gentle." Or, "I don't do asshole play." Do some choreography and role planning. For example, one of you can play the sweet innocent girl, and the other the experienced naughty one. It's easier to invent dialogue this way. Plan the climax(es) of the performance. Set up clear verbal cues, like, "Oh, don't stop, I am going to come!" This way, your co-star doesn't mistakenly stop the action at the wrong moment, leaving you all-too-obviously faking an orgasm. Performance-wise, don't go for delicate subtlety. Grab your own nipples, moan, writhe, and invoke deities in tried-and-true porn-star fashion. This is a visual experience for the client, so plan on playing it large.
During the Show: Both of you talk about how sexy the other is. "Ooh, she's so hot, doesn't she have gorgeous tits/legs/lips/whatever?" Giving the show a context makes it more believable. Tell him you've been wet all day just thinking about touching her. Or invent stories to tell him about other sexual adventures you've had together. Keep wear and tear on your work- partner's body to a minimum by simulating as much as possible. If you are going down on her, let him see it clearly for a moment, then block the view by letting your hair fall forward, or pulling her thighs up around your face. Once the view is obscured, just keep bumping your head around down there, occasionally slurping loudly on her inner thigh. (She should be talking about how good your tongue feels.) For finger penetration, fold your fingers under and put your knuckles at the mouth of her pussy, so it looks as if you are penetrating deeply. Even if you are lovers in real life, simulation keeps you anchored in the reality of this as a performance, and minimizes weird emotional hangovers afterward.
Watch your partner's back. Pay attention to her verbal cues. And be careful out there.