Just from basic knowledge about diet and how it can affect the health of your colon, this question has more answers than one would think at first glance.

If you eat foods that cause one extreme or the other, it is going to be uncomfortable. Example: If you eat a lot of cheese, you can get constipated. This could cause tenderness inside of the rectum and around the anus. Conversely, if you have very soft bowel movements, this causes people to have to wipe the area more, and can cause a lot of anal tenderness. A balanced diet is the key here.

A lot of people like to prepare for anal sex with an enema. It does not really do anything except to get the area cleaned up a bit (note, this area is NEVER truly 'clean', as there are bacteria that live only in the colon). Actually, my recommendations to prepare for anal sex are outlined in my article called "Anal Sex, Doing it Right" that can be found under the "Sex & Love link to the left of this article.