Warning- graphic douching tips are ahead:
Bears are no different than any other men. If you want your ass to be clean inside, then you've have to use an enema or douche. I recommend the kind with the large refillable bag you can buy at most pharmacies or drug stores. You just fill them with warm water (not too hot, though), and insert the smooth tip of the tube up your hole (use lube to do this.)
It might take several bags to clean you out. If you've never done it before, you might find that at first you'll have to squirt the water out within a few seconds of inserting the tube up your ass. As soon as you have the initial shit out of your rectum, you can keep the water going longer for a deep cleaning.
Sometimes excess water can remain, so it's good to use the enema a few hours before you have your ass played with, which allows time for all of the water to leave your system.
I hope this answer hasn't scared you off.
As far as being a bottom, a few words of advice.
Buy a small and a medium
dildo for yourself. Use lots of lube and practice putting them gently up your asshole. You'll enjoy getting fucked by a guy more if you've had a little practice first. In fact, before you have sex it might be good for you to loosen yourself up with these dildos so you're ready for the real thing. And even if he tells you he doesn't have AIDS or any other diseases, make him use a condom anyway. Better safe than sorry.
Okay, now I KNOW I've probably scared you off. Sorry if this is too much info, but it's been the typical path for me whenever bottoming (I'm versatile, so I enjoy fucking and being fucked.) Good luck, my friend. It seems like a lot of work (it is), but if your into it, it can be totally worth it. Nothing beats that feeling of being fucked. Nothing.