I discovered this one just this morning. I post it as a male technique, but who knows? Maybe it would work for females too!
I was lying on my back and had just inserted my favorite butt toy. In that position I usually use one hand to move the toy in and out while I jack off with the other hand, but this time I wanted to use both hands on myself. So I bundled a blanket up against the flat base of the toy to keep it in, holding the blanket in place with my feet.
As I masturbated like this, for no particular reason I started moving my hips back and forth with a horizontal motion. This wiggled the toy back and forth in my butt. It felt GREAT and, with one hand stroking my penis and the other caressing my balls and perineum, made a nice multi-point stimulation thrill.
I enjoy sitting up and moving up and down on an anal toy as I masturbate, but that back-and-forth sensation I stumbled onto was a delightful surprise.