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Here are a few things to remember when you are bottoming:
Regarding cleanliness:
First and foremost, shit happens. If you are embarrassed about the possibility of having a little mess, then you should just stick to oral sex. Focusing too much of your energy on whether or not you are properly clean/shit-free while you are getting fucked will make it NOT FUN. I have never used an enema, but I have bottomed countless times. Sometimes it gets messy, 95% of the time it's fine. I also don't eat food that makes me shit liquid, and I don't let someone fuck me if I feel shit inside of my bowel.
Regarding the pain:
Practice. Practice. Practice. Finger yourself in the shower. Finger yourself in your bedroom. Finger yourself in the morning, noon, and at night. Find your g-spot and jerk off while you finger fuck your prostate.
Most people know this, but just start with one finger, then two, and then start sticking things up your butt if you want. Don't use anything dangerous (I am thinking about that video where the fat dude sticks a jar up his ass and probs dies because of it. Don't do that). Gradually you will get used to two things: 1) Your butt is very accommodating 2) It feels really good
Also, when it comes time to take a huge dick remember to BREATH when it is going deeper into you (assuming you have already fingered yourself and your partner has joined in). Take a deep breath, and let him push in more. If it hurts don't hesitate to tell him to slow the fuck down and wait for the pain to go away for a second. Take deep breaths, and remember to think about how fucking amazing it feels to have a dick pounding you. If you focus on the fact that it hurts, it will only hurt, if you let the pain just be a minor inconvenience then, at least in my experience, it goes away.
BREATH IN DEEP, and then BREATH OUT and let it relax your ass hole.
Anymore questions? I have links to sites that have more extensive explanations, if you are interested.