Real people do not talk in porn cliches.
They do not say: "Give it to me, big boy."
They do not say: "Suck it, baby. That's right, all the way down."
They do not say: "Yes, deeper, harder, deeper! Oh baby, oh Christ, yes!"
At least, they do not say these things to me.
Most of the time, real people say all kinds of weird, funny things during sex, such as, "I think I'm losing circulation" and "I've got a cramp in my foot" and "Oh sorry!" and "Did you come already? Goddamn it!"
Use all the senses
The cool thing about sex -- aside from its being, uh, sex -- is that it engages all five of our human senses. So don't ignore the more subtle cues. Give us the scents and the tastes and the sounds of the act. And stay away from the obvious ones. By which I mean that I'd take a sweet, embarrassed pussyfart over a shuddering moan any day.
You may quote me on that.
Don't obsess over the rude parts.
Sex is inherently over the top. Just telling the reader that two (or more) people are balling will autmatically direct us toward the genitals. It is your job, as an author, to direct us elsewhere, to the more inimitable secrets of the naked body. Give us the indentations on the small of a woman's back, or the minute trembling of a man's underlip.
Don't forget the foreplay.
It took me a few years (okay, 20) to realize this, but desire is, in the end, a lot sexier than the actual humping part. So don't make the traditional porno mistake. Don't cut from the flirtatious discussion to the gag-defying fellatio. Tease the reader a little bit. Let the drama of the seduction prime us for the action.
Fluid is fun.
Sex is sticky. There is no way around this. If you want to represent the truth of the acts, pay homage to the resultant wetnesses. And I'm not just talking about semen or vaginal fluid. I'm also talking sweat and saliva, which I consider to be the perfume of lovers, as well as whatever one chooses as a lubricant. (Sesame oil is my current fave, but it changes from week to week.)
It takes a long time to make a woman come.
I speak here from experience. So, please, don't try to sell us on the notion that a man can enter a woman elicit a moan or two, and bring her off. No sale. In fact, I'd steer clear of announcing orgasms at all. Rarely, in my experience, do men or women announce their orgasms. They simply have them. Their bodies are taken up by sensation and tossed about in various ways. Describe the tossing.