1. Analingus/rimming is a great warm-up
2. Lube, you can never have enough lube
3. Even if both of you are 110% disease free wearing latex gloves is probably a good idea before anybody sticks a finger in somebody's ass. Not only is santorum (the frothy mixture of lube an fecal matter) not something you want on your hands, but the edges of a fingernail can be very sharp and not something you want near an intestinal wall.
4. Use a properly designed sex toy. Or else you run the risk of your x-ray being used at the hospital radiologists Christmas party.
Radiologist 1:
"Is that a rock?"
Radiologist 2:
"No, its a potato wrapped in tinfoil"
ER Doctor:
"How exactly did this Coke® bottle get lodged in your colon?"
Patient:
"Um... I slipped in the shower."
*Both are 100% true cases from the local small-town hospital my sister-in-law is the radiologist at.
Properly designed toys are far less likely to get stuck or cause internal damage.
5. You may want to void your bowels beforehand.