Receiving Reiki is a wonderfully passive experience. Don't try to relax, just let the Reiki relax you. Your state will shift quite naturally as the session proceeds. Meanwhile feel free to daydream, enjoy the music, or simply observe your breath or the sensations of the therapy.
What happens after the session?
Do not expect a diagnosis, as that is not part of Reiki. Some practitioners may make common sense suggestions for after-care, such as drinking water and following your body's needs.
Although people typically leave a Reiki session feeling refreshed, sometimes they notice feeling more tired in the evening than usual. This is not viewed as an adverse reaction, but rather as the body's natural healing response, something to be heeded. People commonly report a sense of calm and mental clarity and sleeping well after Reiki.