Take it from a lunatic who has made close to a grand writing nothing but sexy fight scenes for the past six months: writing sex and writing to arouse are very different skills. You could master the former, but never figure out the latter.
Before we get started, let me say this spiel is an account of things that have worked me, Jub, and is not meant to be taken as an all-purpose guide. Since you are not Jub, and will never be Jub, the actual Jub recommends that you keep your mere mortality in mind at all times.
That said, here are some tips that might help you get readers all hot and bothered.
Look, I'm gonna be straight with you on this one: When it comes to getting the reader off, prose. Is. King. People don't pay me to tell original stories about dynamic characters struggling to find their places in complex worlds. They pay me to get them off. It's porn, not dinner theatre, so quit going for the Oscar and learn how to describe things in interesting, original and provocative ways.
Smut is perhaps the only genre where writing is more important than storytelling. 'Cause here's the thing: nobody is masterbating to the depth of your characters or the cleverness of your plot (and if they are, congrats, you're writing porn EqD pre-readers). Strong storytelling will enhance your smut, so don't leave it out, but you shouldn't depend on it either. Much like with real sex, when the pants drop and the naughty bits come out, you need to perform. And remember this: When it comes to smut, excellent writing can elevate a weak story, but the opposite is rarely true.
Story is important. I know I dis story in smut all the time, but it does need to be there. Readers never just wanna jerk off; they want some adventure, some character development, some twists and turns. Story is necessary, so don't get lazy, but always keep this in mind: Story, ultimately, is not why we're here.
If you think of smut as a scenario, instead of a traditional story, you might be in better shape. Porns have scenarios. A classic example is the pizza guy who ends up fucking a hot college student, or a hot housewife, or a hot whoever. Though this is a weak scenario, if the actors are sexy enough, the camera angles just right, and the actor does "that thing" to the actress (you know, the thing your girlfriend never lets you do), then chances are you're still gonna jerk off.
And that's my point. Story in smut is icing. Nail it and you're a superhero--you've gone above and beyond the call of duty. But shoot and miss? Eh, whatever, that's not why we're here in the first place. Your story can slip a little. Your writing can't.
3. Know Your Audience
Depending on who you're writing for, this rule can potentially negate the first two. Knowing your audience is alway is important, but it's especially important when writing to arouse, 'cause, you know, taste is a thing. If you're writing for whiny lesbians and bronies (no offense lesbians), then your readers will likely bitch at you for not getting their favorite character just right, or call you on some perfectly meaningless inconsistency that no normal person would notice while stroking their own dick. Trying to arouse everyone is crazy, so don't. Instead, know your people and give them what they want. Be indulgent. Pander--go on, this is one time you're allowed to!
I do a lot of commission work, which means I literally have to indulge my reader. I have to give him what he wants, how he wants it. And that's perfect! That's porn, that's sex! Pleasure the reader the way you would pleasure your partner. Learn where his buttons are, and learn how to press them.
(As a sidenote, if you can, write for people with oddly specific fetishes. These people will be so thrilled you are writing their niche, they won't give a crap about your ability, and toss tons of money at you. And before you ask, yes, that is the real secret to my success .)
And finally: 4. Join in!
Why the fuck are you writing porn and not jerking off yourself? Seriously, loosen up, have some fun, write one-handed for shit's sake. This, above everything else, is the most important rule. If you can't get yourself off, you damn sure can't the reader off.
When I first started writing smut on this site, I admit I did it for the views (views I never got, mind you). I wrote things I thought other people would like, but wasn't into it myself. I was frustrated for a while too, because there was nothing I could get off to within my favorite fandom.
Then one day I started writing fics about my own sexual interests. And you know what happened? After only a few weeks of posting fics on FA, someone left me a pretty cool message.
Hi, I just had a question. Do you do any commissions or requests on stories? if so what would the prices be and how long does it take to finish one? I am just curious is all.
See, taking commissions was never my idea. A few people read my work, and one of them decided it was worth money. The rest is current events.