The harder you try, the worse it gets. To write great scenes and dialogue, you need to be relaxed. Even if a deadline is looming. Especially if a deadline is looming. You need to write as if nothing whatsoever depends on what you’re writing. You’re writing because it’s your favourite thing to do. That’s when the ideas really start flowing.
While I write, I like to distract the critical, anxious and agitated parts of my mind with the blandest new-age type meditation music. If you actually sit and listen to this kind of music, it’s utterly boring. But as an accompaniment to a creative (or indeed erotic) activity, it’s the best. I’m no neurologist, but my guess is it’s something to do with alpha waves, left/right brains and stuff like that.
Whatever works for you. As long as you can relax, be 100% with what you’re writing and let what you’re visualizing flow naturally into the text. That seems to be the most important thing. Just as your lovemaking can be ruined by concerns about “performance” or other distractions, so too your writing.
As long as you are completely present in what you’re doing, you have the best chance of enjoying the experience, and possibly being invited to do it again.