There are differences from one hospital/doctor to another as to pre-donation food and liquid intake, bowel cleansing, etc. so be sure you get clear instructions from your hospital and follow them carefully.
Don't forget to include loose-fitting shoes and underwear for your return trip from the hospital, as post-op bloating can make your regular things quite uncomfortable (or unusable) for a while.
Some donors have requested the support of prayers from a place of worship and/or friends and have felt strenghthened in knowing of the prayers and positive thoughts going their way.
Select some music that you find helps relax and encourage you, and bring along a small (cheap) battery-operated CD player. (A recorder can be helpful as well to save your thoughts and feelings while you may not feel up to writing a journal.)
best wishes,
Fr. Pat