If you want to be fisted, it's a common courtesy to clean yourself up ahead of time. I'm referring to an enema, although it's common to hear it called douching. If your taking someone's hand inside your body, you are turning your rectum into a sex channel, so clear out any unwanted obstructions ahead of time.
The depth of your douche depends on the circumstances and the type of play you want. If someone is waiting in the next room, do not take a deep enema. It could take hours to do a deep cleaning. Instead take a couple of cups of water at a time without bending over. In this case, you just want to avoid any unpleasant surprises. Cleaning the rectum is sufficient for shallow play.
If you expect to go for depth or want greater peace of mind, spend a few hours to clean up. Use a water bottle style enema, bend over or lay on your left side to allow the water to flow deep. Evacuate the water. Wait and repeat until the water consistently flows clear. Take a longer break and repeat the process. Stay near the bathroom so you don't have an accident. If you feel daring, involve a partner in your preparation. This is an especially intimate act that can increase the level of trust during the main event of fisting.
After cleanings, have some yogurt with active cultures to replace your supply of good bacteria needed for digestion. Active cultures are key.