Some tips:

Readers who seek out erotica tend to be motivated more out of curiosity and boredom than actual lust. So the goal is not to be explicit in your depictions of sex. Rather, you should endeavor to make the sexuality in your fiction mysterious and surprising. Since even the most peculiar sex acts have already been well-documented at this point, the best way to keep readers guessing is to introduce them to new body parts. The simple addition of a prehensile lobe protruding from the nape of your heroine’s neck or an extra sinus cavity concealed coquettishly in her armpit is all you need to provide your readers with a much needed thrill.

Or you could follow Edith Wharton's lead. In an outline for the story "Beatrice Palmato," Wharton annotated the following as "unpublishable":

As his hand stole higher, she felt the secret bud of her body swelling, yearning, quivering hotly to burst into bloom. Ah, here was his subtle forefinger pressing it, forcing its tight petals softly apart, and laying on their sensitive edges a circular touch so soft and yet so fiery that already lightnings of heat shot from that palpitating center all over her surrendered body, to the tips of her fingers and the ends of her loosened hair.

(Photo via Copyranter, who writes, "Vintage Books should place one of these in front of every display of the BSDM "classic" [Fifty Shades Of Grey]")