The Joy of Writing Sex by Elizabeth Benedict.
This passage sticks with me:
So, I'm thinking about my character Alicia. She's appeared in Betting On Love, No Other Lover Will Do and His Sexy Bad Habit. She's smart, beautiful and the last single girl in her group of friends. So, what does sex mean to her and how long has it been since she's gotten some? When she does connect with her hero, it's going to be epic? Erotic? Emotional? She hasn't told me yet. LOL!
Make sure there are two things happening at once, whether you're writing dialogue or a sex scene. . .It needs to reveal something about them, act as a metaphor, a symbol, or an illustration of an aspect of your theme, your plot, and/or your character's desires and dilemmas.