Learn when and how to say just the right thing. The last thing you want to do is say the wrong thing at the wrong time. For example if you shout out "more!" and he's finished he may be embarrassed and you will definitely be so embarrassed too!
You have to pay attention to what's going on and choose your words carefully, you don't need to really put a lot of thought in to the how to say it but when to say it.
Most couples get caught up in sex and sometimes lose track of their common sense and say stupid things, which may get you a very funny look or kill the mood all together. The last thing you ever want to say is "Are you done yet" or "You're done already" as you can see these are not exactly compliments.
Many woman and men fantasize during sex which is normal and usually the "fantasy" is about a celebrity or a model. You can certainly see how calling out the wrong name even if it is "Brad Pitt" or "Tyra Bank" will kill the whole thing instantly.
In order to keep from offending your lover you should always have his name on the tip of your tongue even if your mind is off with Brad Pitt.
Believe it or not many couples also have problems knowing what to say when talking dirty to each other. If your normal demeanor is quiet and romantic yelling out something raunchy will most likely be a huge turn off for him.
On the other hand if you're generally a little wild one then a lame "Oh Baby!" will also be the wrong thing to say.
The most important thing to remember when you're talking dirty to a guy is to be yourself after all that's what he loves about you, so don't try to be someone you're not!