1. Flattering lighting. Not that everyone needs to be seen with vaseline over the lens, or surrounded at all times by a thousand scented candles, but no one likes to be under the fluorescent overheads. Be kind to your flaws.
2. Not having just eaten/drank some wine. Way too many dinner dates make for the post-eating sleepies which prohibit the more vigorous kinds of sex.
3. Feeling comfortable with the person you're sleeping with, and not as though you are only being used for what you can offer sexually. No one should feel like they're constantly at risk of getting kicked out for breathing wrong.