Just when you think you know pretty much everything there is to know about sex and your bedroom looks like you played supermarket sweep through Ann Summers, your girl throws that awkward curveball and whispers 'talk dirty to me,' in your ear. If this catches you off guard you can find yourself naked, mumbling 'I'm going to umm put my umm you know, inside your, umm, your ooh what do I call it, a, sorry, ha.' or even worse, just shouting the word 'sex' over and over again because you don't know what else to do.
Anything you say can sound dirty if you utter it in a certain way. Think of the voiceover on Masterchef. She says everything like she's having a continuous orgasm, and if she actually is that's commitment right there. Even a phrase like 'pasta salad with cucumber on top,' can sound extremely filthy if said in a quiet breathy way. I mean I wouldn't suggest you say that, it might kind of ruin the moment but you get my point.
If it's too embarrassing to say certain things or you feel stupid then develop your own names for things. If she does something that you really like it then label it your favourite, or ask her to 'do that thing' because that could mean anything from touching you to covering you in cake mixture and licking it off. Don't say 'the usual' though because that sounds like you're ordering a pint with a side of oral sex.
There's talking dirty and then there's insulting someone. If your girl is sobbing in the middle of the bed, then refuses to talk to you for two then chances are you did it wrong. Separating the fantasy in the bedroom from the world outside it a very important bit. Also if one thing, like calling her dirty or referencing star trek and Finding Nemo worked for one girl, it may not work for another and she may be offended or think you're weird.
Being part of a fantasy or planning out a storyline can help because you're then talking as another person rather than you. You can be anything, a pirate, a princess, a penguin or something else that doesn't begin with P. Go crazy. Another thing to decide is to make sure you use words that you are both comfortable with. And find out before you say the word 'pussy' and your girlfriend runs screaming from the room.
The thing is to try things out. It might not even work for you. But for the love of God, do not use words that make you want to gag, like flaps, oozing or the word that makes-me-want-to-hurl-myself-off-a-cliff-rather-than-hear-it-again - 'Moist,' This will ruin the mood on so many levels you might not have sex again. Ever.