Important: Having unprotected anal sex is the leading venue for the transmission of the HIV/AIDS virus. Before any type of anal sex activity, it is imperative that you take proper measures to protect yourself and your partner.
Preparatory Bowel Movements
It's a good idea to have a bowel movement several hours before anal sex (if possible). This will vastly decrease the liklihood of running into something unpleasant and will clean out most of the residual matter in the anus.
Cleaning the Anus
At the very least, make sure to clean the anus thoroughly with hot, soapy water before any anal activities. You can even use a finger to massage soapy water slightly inside the rim of the anus. This will also help to relax the outer muscles of the sphincter.
An anal enema is essentialy a surge of water into the anus that looses residual matter and flushes the area. While a full anal enema will certainly be able to clean where you're otherwise not likely to reach, they're fairly uncommon overall. However, there are people who find anal enemas sexually stimulating. If you enjoy it, or can at least handle it, they're a good idea to reduce residual matter. Allow at least 2 hours before anal sex for the anal enema to have full effect. Also, it's not good to perform frequent anal enemas as this can put undo stress on the anus.
Enemas do NOT eliminate ALL bacteria and germs from the anus.
The entire anal region is rich in a variety of bacteria that, suffice to say, belong only in the anus. You don't want to contaminate other areas of your body by inserting ANYTHING that has been into the anus into ANY other orifice of the body.
Due to the ease of transmission of STD's, including HIV/AIDS, condoms have become an essential part of careful anal sex. Take care to use water based lubricants with latex condoms - an oil based lube will destroy the condom.
Source: Female Sex Guide