If you're having trouble getting started, try just talking about the things that are happening. "I love the way you touch my pussy" "Your cock feels so hard in my hand" "I'm so wet" Keep it simple. Profanity is an easy addition too, particularly if you don't use those words often. Throw in a few fucks, cock, maybe a **** or pussy if you don't use those often. And I agree with the comments regarding size. Tell him his cock is big. I love it when my wife does that. I'm not insecure about the size of my dick, but it's still awesome to hear while I'm pounding her.
A lot of times for guys they love hearing how much you NEED their big hard cock in your tight lil pussy or in your mouth and other things along those lines. I think it's the whole feeling that they have this girl who is wanting them soooo badly that really turns them on. It's kinda easy to get them going just by looking them in the eyes and saying "I want you inside me NOW!" it's not that dirty yet it REALLY gets them going. Trust me! I've always enjoyed dirty talking... sometimes I'm surprised with some of the stuff that comes outta my mouth.
watch a porno and say what the girl says. i'm a guy. trust me it will work. it doesnt even have to be clever.
Wait, I call BS. You can't just leave it at that. I'm sure that I speak for the other when I ask, well, what did you say / do? What made it incredible?!?!
http://www.hipforums.com/newforums/showthread.php?t=388012&f=65 ...go there for the back story...I took the advice of a few posters in this thread. I used lines like "you're cock feels so good inside of me", "fuck me harder", "you're so big", "I feel so full", "fuck! you're so hot!", "keep going, baby! I want to feel you cum inside of me"