Now! On to some of my own personal tips. These are specifically in the arena of mutual (hurr hurr) roleplay. The articles above are great for writing sex in general, but what about when someone is on the other side of the keyboard?
Firstly, the advice to not be overly descriptive persists. I once had a guy page me (our characters were not in a relationship and we had never OOCly discussed such things) to inform me that he had rolled dice to dictate his character's dick size and the dice had 'decided' that his dick was the length and girth of two soda cans stacked on each other.
Shall we break down the 'NO's here?
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Unsolicited sexual advances suck IRL and they suck online. There's flirting and there's just being outright crass. Being crass is not going to lead to anything.
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No real (ETA: by real I mean 'realistic and not the type portrayed in porn') woman is going to be turned on by that. Nor interested beyond the clinical 'this should be a museum piece' aspect. That size of a penis is horrifying and I cross my legs just thinking about it.
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There is never, ever a need to describe a character's genitalia in explicit detail. Women don't go on long diatribes on comparing the size of their breasts to canteloupe and how their areola are the size of half-dollars and the hue of pink roses while their nipples are pert like erasers (wink wink to the folks that read the first article linked above).
Vague is good. Vague is good because it lets the reader fill things in. I once had someone describe that character descriptions should be like a woman's skirt: long enough to cover everything, short enough to be interesting (aka leave something to the imagination). Use the same concept in your TS. Imply, be concise, be accurate, but don't be overly/overtly descriptive. Allow your partner to build the scene in their mind without being bogged down by details.
Secondly, don't take forever. This is more directed towards people who play male characters. They've done studies and found that the 'sweet spot' for the actual intercourse part of sex is around the five-to-six minute mark. Pop culture may have some of you going 'Is that it?!', but as a woman: that's perfect. Too much longer and you start to get bored or worry about friction. The number of women who can achieve orgasm from vaginal stimulation alone are very, very few. And the number that can do so every time is even smaller. That's not to say it isn't fun and a fulfilling part of the whole process, but the biologically male get a whole lot more out of it (I have been the awkward third wheel in an argument between a gay couple in which one got fed up with always being bottom).
Think about those combat scenes that just drag out for hours. Or that feeling when it looks like yet another night of RPing socially in the bar. After a while, you start to wonder how many times you can pose the same sort of thing. This goes for TS scenes, too. I can only pose squirming and moaning and raking nails down a back so many times before I'm cooking dinner between poses. And no, I'm not kidding. I have most definitely cooked dinner while TSing before.
Some of my best TS has been over in a handful of rounds. The action was well-written, summarized, and the focus on the characters and not the dirty act of it. When I play a male character, I try to keep things concise and involve the woman, too.
Which leads into point three. Seriously, stop ignoring the woman. Don't get to the 'prize' and just forget about the rest of her. Not only is it going to totally ruin the 'oh we're so happily in love' aspect, but it means she has even less to pose to. And no, spending a single pose in going down on her doesn't count. Describe how your character grabs her hair. The way their hands smooth over her breast or hip. The way your character arches their back and positions themselves to reach her clit. We all hate when we have nothing to 'work with' for our own poses, so keep that in mind. Yes, even when you have just one hand on your keyboard.
You can make things so much richer by making sure your partner has the opportunity to be fully engaged. This means, too, if you're pleasuring yourself IRL... Wrap up the TS once you 'finish.' The other person can tell, I promise. If it goes from vividly-written scenes of lust to bland play-by-play, we know.
These are just a few of my personal thoughts. I welcome others to chime in. Like I said in the first post, try to keep things relatively chill. I know this is MSB, but man if we can make people better TSers by giving genuine answers to their questions, won't the world be a better place? 
