I rarely fake orgasms because I don't believe someone who is fucking you poorly deserves that kind of payoff. More importantly, faking an orgasm is the biggest dick move you can pull on another girl. A faked orgasm during shit sex only serves to perpetuate a guy's misconception that he has "moves" when he doesn't, which he'll likely use on the next girl unlucky enough to fall into his bed. By faking an orgasm, you're inflicting a disservice to sisters everywhere.
That said, there are occasions when it is essential to fake an orgasm like when a) love or intense "like" is involved (ew); or b) the guy is doing a seriously good job, and it's your purely non-physical disposition that's in the way of coital finality.
I should point out—because even grown men don't yet seem to grasp this concept yet—the path to orgasm relies as much on mentality as it does on physical acts. So you can fuck all the right buttons, and hover just near or around climax for a bit, but unless your mind is completely present, it's not going to happen. That doesn't mean the sex isn't lovely. It just means it's been a long ass day and everything is distracting right now.
Here's how to fake an orgasm when the penis in your vagina deserves it most, but you have that instinctive feeling that it's just not going to happen tonight (or this morning, or this afternoon, or whenever you happen to be fucking)...
Baby, Arch Your Back
OK I got this one out of a Miguel song, but if you're going to fake an orgasm you may as well make it as cinematic as possible. When you're having a real orgasm you are so removed from the moment in your ecstasy that you might not concentrate on the inherent sexiness of your movements. Use this opportunity to inhabit those moves. Writhe around, push your hips up (or down if you're on top), and be all lithe and serpentine, the way movies would have you believe people look during sex. BONUS: The dude fucking you will be so enraptured by your alluring posturing that he probably won't even notice you're faking. He'll think he's hit the jackpot, which he has, duh.
Lips are another part of your body that you can luxuriate in your deception. You've probably never seen your face mid-climax, but I'd bet good money it looks similar to the face you pull when you're struggling to lift weights at the gym. Pout your lips, half close your eyes in that hooded, seductive bedroom way, and grab your own hair like a stripper mid-dance (where it would usually be matting into dreadlocks at the back by this point, flick it about like there's a fan in front of you instead, but not too much, because you don't want to look like you're having a fit).
Flex Your Vocal Chords
An old roommate of mine was once loudly fucking a girl in his room one night, and I texted my friend that she was faking it for sure, so manicured was her coital wailing. I'm convinced, when in the throes of passion, most people sound like a bush pig that's been speared through the leg. A faked orgasm is the perfect opportunity to sound like a sexy voice over in an Usher song, which is, if you were curious, one of my dream jobs. When you're ready to Climax, make sure to yelp "I'm coming, I'm coming!" in a husky tone, punctuated with perfectly curated "oohs" and "ahs".
Shake It Like a Polaroid Picture
When you're "arriving" he's got to feel it in your body. You can't just lay there like a gimpy starfish and expect it to be believable. I can't make my knees and hands shake quite the same way they do after a great orgasm, but heaving your bosom dramatically should do the trick. Rub your tits a bit too, guys like that. And you know that secret exercise Oprah tells you to do while you're sitting at your desk to keep your vag nice and tight? This is why. The only reason pelvic floor exercises exist is as practice for orgasm faking. Clench and unclench; MAY THE FORCE BE WITH YOU.
Punch Him in the Face
If you lose confidence in your acting ability, just give him a swift knock to the eye. This is the ultimate misdirection. He'll be so sore and pissed off at you he won't even remember that he had any suspicion you were cheating in the first place. Don't punch too hard though--you don't want this to end in a trip to the hospital, the aim is to get him to roll over and at the very worst go to sleep mad. This is a high risk-reward proposition, because he might be into it, but if he's not, you get to blame your arm spasm on "being in the heat of the moment," and—congratulations—you just faked an orgasm without actually faking an orgasm.
Pant, Say "Wow!" a Lot
Once it's all over, lay about slapping the back of your hand to your forehead like a period drama queen in a too-tight corset saying things like "Wow" and "Oh my God" and breathing deeply. My preferred post-orgasm state is to be apart and untouched, but if you prefer cuddles you can do that too. If you're a good actor, you've just faked an orgasm. Feel free to fall asleep now; after all your hard work, you deserve it.