Oh, how I enjoyed doing this in my early years! It was all about stretching and letting your back loose so it'd bend just a bit more. "Sadly", I grew up and discovered the pleasures of sucking somebody elses (and getting mine suck by them) and I stopped.
Now I'm not as flexible as I used to be so I don't do it that much, but not gonna lie, my bf and I have shared my cock so he'd lick it from the side while I fuck my own mouth. Let's just say cumming in my own mouth and then having my bf suck it all out of it is more than worth the back pain ;P