I'm by no means an experienced writer, but Luna's advice seems solid to me.
When I wrote all the content to my Touhous Triology RAGS games, I wrote something, read it, rewrote it, read it, rinse and repeat. Every time you read it, if something feels off or you think something can be improved, you change it. Doing this over and over again allows you that polish that some more experienced writers seem to have in their works, to an extent.
I think the hardest part about writing erotic fiction, as I found out in my RAGS sequences, is not sounding repetitive. How many different words do you know for each part of the sexual anatomy? If your erotic text relates heavily to sex, you'll be using words for those a LOT...and there are only a handful.
Pointless, but interesting, note: I once found a hobbyist author who had a (very realistic and extremely well-written) work submitted to MCStories. He didn't have the fetish and had no interest in it. So why did he write for our kind? To improve his ability to write generally, and erotically. He found that we, as a community, are more likely to criticize his work for its flaws rather than its style, unlike literary communities as a whole. I wish I remembered who this is or what story it was. He'd probably be willing to offer some tips...