As much as I love WAM sessions, I will admit it kinda scares me when I'm standing in my bathtub and I almost slip and smash my face against the wall. No injuries so far (knock on wood), but I am curious as to how you guys manage not to slip everywhere during your hunger sessions.
I always wear socks. It doesn't always suit the situation but adds so much grip. Plus I find that sports socks are great for wiping down the tub afterwards.
Don't stand up. I just sit until I and the mess is mostly gone, and don't attempt standing until I am 90% clean. A quick shallow bath for the bulk, then the shower is faster. It means making sure everything is set up in arms reach, but for the sake of cleanup that's a good idea anyway. Obviously this wouldn't apply if you tend to get messy on a tarp or pool away from the bath.
You're kind of going to, it's slippery after all. Look around the forum, there is an old thread on WAM injuries with stories even from pro producers and models. Aside from what others have already said above, keep some certified slip-resistant footwear near by. Like the professional grade they use in kitchens and workplaces with oil etc.