I've found that when I'm having my best orgasms, my fingers and toes are clinching. And having my hands free, sometimes I will squeeze a pillow or whatever is available, with the exception of my penis, because that will kill the super Os quicker than anything.
Anyway, I always keep a rag for clean up purposes. I twist the rag into a sort of loose rope and squeeze it.
It makes things better. I don't know why, it just makes things so much easier, longer lasting, and smoother. By smoother, I mean not so much violent spasms, but a gentle continuous wave of pleasure.
And its like I've associated the squeezing of the rag / rope so much with orgasms, that merely doing the squeezing seems to induce the orgasms.
Maybe I'm just incredibly lucky, but I think this is worth trying.