There are several things that you can do to prepare for your Reiki session that will increase your comfort level, as well as the effectiveness of the healing session.  The basic premise behind this is mindful awareness of the experience that you are about to have.  Following these steps will also help your Reiki practitioner feel at ease and comfortable practicing with you.

To begin with, take a long relaxing shower the day of your treatment.  The hot, cleansing water will help refresh and relax you, both mentally and physically.  Avoid using any heavily scented products during or after your shower.  Wearing strong scents could later prove detrimental, as the smell might distract you or your practitioner during the Reiki treatment.  

Dress yourself in comfortable, loose fitting clothing.  Avoid wearing tight clothing, as they lessen your chances of relaxing fully.  Likewise, items such as short skirts should be avoided.  Whenever possible, wear shoes that are easy to remove, and dress in layers so that you can be dressed appropriately for the room's temperature.   Leave the jewelry at home on the day of your treatment- wedding bands are usually fine, but watches, bracelets and necklaces should be left someplace safe and out of the way.  Some practitioners believe that jewelry can interfere with energy flow, while others merely find it to be in the way.  

Be sure to arrive a few moments early for your appointment to avoid the stress of feeling late and being rushed.  Be sure to communicate with your practitioner.  Let them know what your preferences are (music, no music, dimmer lighting, etc) and alert them to any areas you would like them to work on.  This communication is also important during a Reiki treatment.  For example, if you started off your treatment very cold and were lying under a blanket, and later became very hot, it is far preferable that you simply let your practitioner know, rather than lying there under a blanket, sweating.

Each practitioner of Reiki is different, and may have their own specific guidelines for how they prefer their clients to prepare for a session.  The above is merely a general guideline.  Always follow the instructions of your practitioner.