I have been masturbating this area for years and this is how I do it. I clean out my rectum with a warm water douche bulb. Since I like it really clean, I often have to do this several times.
I like using a dildo that has a skin-like feel. If curved the better. (Pre-heat dildo in a pitcher of hot water.) I insert the dildo pointed upward toward my upper butt crack, since the rectum inside is also curved this way. I usually sit astride the dildo with a towel and a hard pillow underneath it, this help to bring it up higher towards my anal spot opening, then I glide on K-Y, a lot and I bend forward doggy style and get my rectum really juiced up. Stroking wildly, I keep the lube in by putting on the tip and try to dilate my rectum and slip it in. I proceed to the pillow and sit on it, writhing and churning my hips on the sex-toy. I feel intensity build up and a tightening in the rectum length and I orgasm rectally, it is intense and goes from the groin to the back up to the top of my head/brain area in a flushing and wave like sensation, once relaxed enough, I stroke on the dildo.
Now here is a twist, I buy one of those silicon mega-stretch pleasure rings from the sex store and I place it on the dildo about 1-1/2 or more inches down on the shaft of the dildo, I like the ones that have tendrils or small 1/4 inch nubs all over it, now the trick is to get it in. Your anus will need to well stretched by then, so it hurts very little. Once in it is pure ecstasy. I then hump up and down on the dildo with the pleasure ring on it and I can feel more girth in the certain spot inside me, I will let the pleasure ring come close to coming out for a while then I can be stretched more and I let it all out and in again.. The intensity will build and I orgasm over and over again, at least 30 times if I want or more... I can go for hours until my legs are tired or I jerk off and cum to end my session of anal eroticism. One thing I need to mention, the pleasure ring has come off a few times, don't panic! I have defecated it out the next day easily, but if you're careful you can reach in with a index finger and feel around inside and you and get it out that way. Be sure to have clipped and filed nails. I have found if I dry the dildo off in the spot where I want the pleasure ring and put it on then lube it up, it rarely comes off. Another way is to place a condom over the whole dildo and P.R. and lube up the condom..enjoy...you will never be the same! p.s. I'm a heterosexual who has a high sex streak for novelty and sensual delights, I need to tell my girlfriend what I am like in this area, hope she does not reject me. Also I use 2 mirrors as I fuck my dildo, you will have to be creative as to how you accomplish this, but a chair or table and tape the one mirror behind you tilted down a bit and waa-laa, your like in a movie like feeling of being watched as I gaze into the one in front of me a few feet away tilted toward the rear mirror catching the reflection of me and my anal area seeing it slide in and out...