Its a matter of relaxing your muscles and being felxible enough to do it. It may not come naturally but with practise -and no strain - it can be achieved. Best way to do it is to lay on ur back and 'walk down the wall' I find. Also, dont ahve ur head as near the wall as ud think....experiment a bit. Once ur body gets used to that ull be able to increase ur flexibility and do diff thigns....
honestly, I find it more of a physcological thing. Its more the thirll of knowing 'i can suck my own cock' rather than the feeling it gives you. dotn get me wrong, its damn good!!! But still, its like J/o - sometimes its better to have someoen do it to you..