I may be no more qualified than others who write sexy stories and post here; but I figured I would share what I have learned from reading and writing erotica for many years.
The first and most obvious point is that erotica, unlike other kinds of writing, has a purpose and that is --to get the readers sexually aroused. I like erotica with a good story, sympathetic characters and colorful descriptions; but the story also needs to have strong sex scenes. Personally, I am more turned on by scenarios where the writer has used his or her imagination and stretched their vocabulary a bit. Falling back on generic words like 'cock', 'dick', 'cunt', 'pussy' with handy adjectives turns people into puppets.
Know your audience. This means thinking about who will be reading your story. "Different strokes for different folks," starting with the fact that, in my experience, women and men are turned on differently. Men's default setting is visual:
"She strode into the room boldly in white thigh-high boots of shiny patent leather. Her firm cheeks wobbled with each step."
Women's default setting is more likely to be on what they hear and they listen for how people are feeling.
"She made a point to stride into the room boldly; knowing the women would hate her Ferragamo thigh-high boots with four-inch spiked heels. She didn't care. All the men would be scoping out her gym-toned ass." If you are appealing to women readers take your time with descriptions.
Descriptions and storytelling are foreplay. Don't be in a hurry. Tease. It is sexier to keep the couple apart and wanting each other than it is to rush to voracious tongue gobbling.
The genre you choose to write in Sy Fy, BDSM, LGBTQ, Mature, First Time, Incest ... is going to change the kind of language you use to describe your scenes. People in those interest areas tend to be different ages with different levels of experience and of course different genders. It is fun to try your hand at some kind of erotica that maybe isn't your affinity group. Do your research. Know your vocab. Do you have to experience a group grope before you can write about an orgy? Maybe not; but you should have read enough stories of that ilk to know the ropes.
Don't be a copycat. Think up something new or original to set your "nubile by her uncle's pool" off against a hundred others. What are some ways she appears different to him? Avoid bust measurements. Imagine a real person. What can she say that fascinates him? Why would she be intrigued by this older guy?
Have a plot; something other than how they get in the sack. What scares her? What scares him? Is there a job they have to do together? Does either of them have a secret?
How can the sex be unique? What can your characters do with their lips, teeth, tongues, fingers, knees, elbows? How detailed can you get in your description? What might he do with that heart-shaped mole just inside her left hip-bone?
