Your body holds feces higher up in the intestines until you're actually ready to "go," so chances are you won't encounter much. How do you get a cucumber out of an anus? Completely removing the penetrating object and giving the anus a couple of seconds to chill before sliding back in often seems to convince your bum that whoever's knocking on the backdoor is a friend and it's okay to let them in. Start off with 10 or 15 minutes of plugging and of course be prepared to take it out sooner if it hurts. You have to simply work your way up with bigger objects until the act that you are trying to perform isn't extremely painful anymore.
It is all about starting out small and working your way up.
How to train your anus