SP suggested starting a thread about this, so that's what I'm doing. And yes, this is a mature, sensible thread, though you can probably count on SP and me to throw in some silliness here and there. Seriously, though, I'd really like to keep this somewhat on topic, as I think it may be a valuable discussion.

Basically, this thread is about observing sex in books and film, observing how sexual tension is created/maintained, etc. What scenes have always stuck in your mind? Which fell flat? What made them stick in your mind or fall flat?

Post links to YouTube videos (I think it's safe to assume most are NSFW in this case) if you have them, or at least describe them as best you can (which in and of itself is a valuable exercise).

I posted this in another thread, but will repost it here: Last summer, Scarletpeaches and I spent several hours every day for about 2 straight weeks analyzing the hell out of sex scenes in both book and film. (YouTube, we love you...) We'd read/watch them and ask ourselves/each other why a particular scene worked and why one didn't. Why did one fall flat while the other created visceral, physical reactions (aka, tinglies!)? What little details made our breath catch while others barely registered.

To get you started, I highly recommend (obviously adult content, NSFW, etc):

From "The English Patient":
http://www.youtube.com/watch?v=kUrRAF-RwNw

From "Atonement" (this is easily one of my favorites; There's something heart-stoppingly sexy about a kiss or a touch that says "I need you NOW". To me, that's what makes this scene so sexy...all the tension and buildup, then a "can't wait another second" release. Especially take note of her skirt going up her leg, the strap from her dress between his fingers, things like that.)
http://www.youtube.com/watch?v=izs4_0X7UlY

From "On Becoming Jane" (this isn't a sex scene, but is an incredible example of romantic and sexual tension. Watch the whole thing...it's slow to start, and the part I'm thinking of is at 2:08, but the rest of the clip builds up to it. It's really little more than a look shared between two people, but it speaks volumes. And the fact that James McAvoy is hot doesn't hurt, but I digress...)
http://www.youtube.com/watch?v=DDqZsNV_7bY

From "Mr & Mrs Smith" (particularly from about 0:10-0:27 - their eyes speak volumes; the rest of the clip has its pluses and minuses; I don't recommend having your characters fucking on broken glass):
http://www.youtube.com/watch?v=CSfZHFJt_5E

From "The Fountain" (skip ahead to about 2:15; 2:45 on is especially good):
http://www.youtube.com/watch?v=zOCUzLA7BGU

From "Out of Sight" (I don't care if you hate JLo as much as I do...this whole scene is sexy.)
http://www.youtube.com/watch?v=AWXC1NlER4k

From "The Matrix: Reloaded" (Obviously something like this is going to translate better to film than to the written word, but it's a good example of using the environment/surroundings to emphasize the feelings of a scene. There's an incredible, primal rhythm to that scene, and while the movie itself was mostly made of fail, that scene was spectacular.):
http://www.youtube.com/watch?v=wdj3SzQrCII

Other:

True Lies - Striptease scene. Pay attention not to what she's doing, but how he's looking at her. Even though you can barely see his face, the looks are there.
Shakespeare in Love - SP will have to narrow down which scene it was, as I haven't seen the whole movie, but there's a particular scene when the character played by Fiennes gives Paltrow's character a LOOK. Just watching her across a room, and his face has pure longing written across it. (see? It's not just sex scenes...any kind of interaction, tension-builders, etc)
The Recruit - Parking garage kiss, spectacular example of "must have you RIGHT NOW" tension.

Remember: The tension does not begin or end in the bedroom. Putting aside my undying hatred of the movie Alexander, and my insane crushes on both Jared Leto and Colin Farrell, there is one thing that makes that movie worthwhile: The sexual tension between their characters. They never do anything onscreen (Oliver Stone, for this I hate you), and their dialogue is horrible (hey, at least it was consistent with the rest of the film)...BUT the looks they exchange. Good God. If anyone can put on a look of sexual longing, it's Hephaestion when he's watching Alexander marry someone else. They both nailed the chemistry and sexual tension.

Figure out what makes those scenes sexy, what gives them tension, and translate that onto the page. It's not easy, but trust me on this one. After SP and I did this, our sex scenes and the sexual tension between our characters changed dramatically.

And while you're at it, read my rant about how not to write sex scenes.