I dont know how you guys do it. I have a hell of a time getting a thin dildo up there let alone anything larger than cock size.
You don't just do it. You have to work up to it, sometimes over a period of years. The sphincter can be stretched, like any other muscle, but it takes practice and time.
Of course, you have to really
want to do it, which usually means you have to be turned on by the idea of taking something large inside your ass. If you believe that it's going to be difficult, painful, messy, or otherwise unpleasant, you're not going to succeed and you're not going to have much fun trying. If, on the other hand, you believe that it's going to be intensely pleasurable, exciting, and fun, you're at least in the right frame of mind to give it a try.
It helps if you're very aroused before you begin. I don't know if it's because of the endorphins or what, but you can accomplish a lot more in a state of intense arousal than you could do otherwise.
Paradoxically, it's also essential that you learn how to
relax. While you are experiencing an intense psychological thrill, your body (well, your muscles--your heart may be pouding fast) should be in a deep, almost meditative state of relaxation. It may help to have some prior experience with meditation, yoga, martial arts, or some other discipline that requires a calm yet intense focus. Being deeply relaxed while simultaneously experiencing extremely intense sensations makes the experience almost transcendental.
Do other bottoms agree or do you have a different perspective?