It really depends on what all you're planning on in your experience. For me, I have to have a confident clean to really enjoy the experience. The thought of accidentally making a mess during an otherwise hot session gives me great anxiety. Sure, it's all part of the territory, but I still much prefer to take some preparations and alleviate those concerns.
This is my experience, and it may vary for yours, I like to be very mindful about my "cleanliness" leading up to an anticipated anal experience. A lot of it depends on what all I am expecting... I like deep penetration, which requires the most preparation.
First, be mindful of what you eat the day before. You sure don't wanna go eat a massive greasy burger, chili cheese dogs, etc...for obvious reasons. The day of the planned encounter, if I'm planning on getting to business at 9pm, I'll try to eat a nice breakfast, a light lunch, and then forego dinner in lieu of a snack. About 5pm, I will begin my enema flushing process. First, empty your bowels. Then, hop in the bathtub, and get one of those hot water bottle/enema kits, and fill it up with warm water (no soaps, no cold water as it causes cramps), put a little lube on your hole and on the enema tip. Lay on your left side and insert the hose, and let the water go in slowly. You have 2 sphincters in your rectum--one voluntarily controlled, one involuntary. In addition, about 8-10" into your colon, you have another involuntary sphincter (I've heard anal enthusiasts call it "Heaven's Gate") that is (I believe) at the entrance to your sigmoid colon. If you're planning on your experience going that deep, then you need to plan on flushing this segment too.
As you lay on your side and let the water fill you, and if you pay close attention, you'll feel when the lower section of your rectum up to the beginning of the sigmoid colon fill up. When it is, stop the water, and try to lay there and hold it for a couple of minutes. Then slowly get up, sit on the toilet (while putting your feet on a small stool if you have one, to get your knees up higher) and slowly empty the water. If you push it out, you may find yourself irritated by the end of the process.
Repeat this several times. I usually go for a thorough flush, so I make sure water makes it up to the sigmoid as well. When you empty and see clean water, then get a long, flexible toy that can push past that sphincter at the sigmoid to make sure there's not any dirty water trapped up there. After you're satisfied of your cleanliness, shower as usual. Afterwards, I take an Imodium AD, as it's function is to remove water from your bowel...
It may seem like a huge hassle, but it's totally worth it to me.