This information is based on years of personal experience specializing in pegging & prostate play. I am not a certified healthcare professional.

Right before our session:

Whether outcall in your space or visiting me at my incall, please shower before our time together. Using soap, wash all over to take care of sweat and body oils that have built up through the day. Pay special attention to your genitals, including your asshole. Use soap to clean your butt, including your butt crack; if you can, use your fingers to clean the opening to your ass, including with soap.

Enemas
If you want to go the extra mile, including during our session and major ass penetration, consider doing an enema 1-4 hours before we meet. You can buy a simple enema kit from a pharmacy or online; if you want to go hardcore there are shower attachments you can buy on Amazon.
If you are looking for a sex-positive and non-judgmental store to buy enema products, I recommend Little Sister's Bookstore at 1238 Davie St, Vancouver. I'm a fan of their anal douche bulb products, which are easy to use.
Here is a link to a 14-minute tutorial about enemas - jump to 4:55 in the video to bypass the ambiguous hippie talk about "toxins" and get to the practical info: https://www.youtube.com/watch?v=-UMu_iP-AJE
For best safety, use distilled water purchased from a store, or water that's been boiled and then cooled to room temperature (not plain tap water) for your enema.


The day of our session and a few days before:

Eat lightly, sticking to smoothies, soups, salads, or maybe a light pastry. Avoid large meals that include meat or large amounts of carbs, like pasta. Hydrate well - have water, keeping coffee and alcohol to a minimum.


To maintain your digestive system between or in advance of sessions:

Eat a healthy diet inspired by author Michael Pollan or the Canada Food Guide: eat food [REAL food, that your grandma would recognize], not too much, mostly plants.
​The Canada Food Guide recommends that half your plate is produce, a quarter carbs, and a quarter proteins and that you have water your meals. This helps with fiber and water in produce that will keep your digestive tract moving and nicely scrubbed, without constipating meat and carbs clogging you up and without diuretics (that make you go pee), like caffeine and alcohol, dehydrating you.
One easy way to do this in real life is to always order a glass of water and a salad when you eat out and make sure you consume them before anything else. Even if you have a burger and a beer with your water and salad, you'll still get what you need to help keep you and your digestive system healthy.