OK, I've had lots of experience with this... You don't want to do enema, but that's really the best way. We don't have enough control over our "processes" to trust that just eating certain foods the day before, taking a crap (how crude, lol!) the day of, etc., will be enough. And, if you use an enema, you don't have to do anything special the day before of day of, so that's a definite bonus.
I experimented and found the best way was: (the enema instructions from other reply are good but only if you have just the right kind of shower head, water pressure, removable, etc.)
So: Use or buy an empty squeeze bottle, like you'd use for shampoo, conditioner, etc. Something that holds plenty of water and that you can easily hold in one hand, and that you can squeeze easily to dispense water from the opening.
-- Get naked in the bathroom, turn on the shower to desired temp., and then fill the bottle with warm water from the sink, or from the shower.
-- Get in the shower, with the bottle, (you might want to lube-up the opening of the squeeze bottle before this), get on your knees and hands (you may want to wait to turn the shower on if the shower water getting in your face or whatever is a prob), and insert the tip of the squeeze bottle opening up your ass, lol! You want to have your ass sticking up in the air when you do this part. No, you WILL want to...
-- Squeeze the warm water out of the squeeze bottle, up inside you, and I'd squeeze out as much as possible.
-- Stay in the ass-in-air position for a couple minutes. You have to keep your ass in the air so the water will go all the way into your lower bowels and get EVERYTHING nice and cleaned out!
-- When you feel that the water has worked it way as far as possible into your bowels, you need to quickly stand up and: either squat right there in the shower, or quickly step over to the toilet and sit on the toilet.
-- You'll naturally find that you can squeeze the water out of your anus, which is both weird and enjoyable, it's like peeing out your butt! You can repeat this as often as you feel necessary, you'll know by looking, and feel, when you're totally cleaned out.
-- This kind of enema makes anal totally fearless as far as mess, and it really helps to be able to let go... because we unconsciously squeeze our rectal muscles together, holding in anything that may come out (unless you're just totally uninhibited, which is rare)... so being totally cleaned out like this gives you the confidence to just relax those muscles during sex, and you have the confidence that there's nothing up there that will get in the way or cause pain. As for the squatting in the bathtub the to expel the water, usually you'll have "moved your bowels" (taken a ****) before you do the enema, so all that will be coming out will be liquid poopy water and then clear water, so it's not as disgusting as it may seem to do that, and it's a lot easier than stepping over the toilet! Enjoy, hope you're helped by some of the suggestions!