Wash your arse scrupulously with soap & water as close to the booking as possible, as well as showing willing by also having a quick shower at the WG's premises. Push down with your pelvic floor muscles with the shower blast up your chuff to reach every nook and cranny.
Then get ready for one of the most intense cums of your miserable life 
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Dito above + the following pro tip: take a bottle of water with you which has one of those little spouts for drinking through. Screw that into your balloon knot and squeeze a quarter of contents up there to flush anything nasty out of your rectum (obviously do this over the loo and not while you're in the shower). Some WGS might insist on doing a finger test before engaging in anal play and if you do the above you'll get top marks for preparation.
However, pump too much water up there and you'll flood the sigmoid which will create one hell of a mess. If you start to feel stomach cramps then you've gone to far and rimming (along with everything else) will certainly be off the cards until you've done a complete and thorough flush out (with a least a couple of litres of water), otherwise a single unexpected fart could pebble-dash the surrounding area. Might be an idea to practice at home first with this sort of thing.
Glory awaits
Edit: this thread has piqued my interest in Madam Koko. Thanks for the lead.
