yoga, and other excercises that increase flexibility. it's wayy easier to find someone else to suck it for you.
In my younger days I could do it - more out of curiosity than from the actual satisfaction achieved to be honest. The way I started was where my bed was against the wall I would stand on my shoulders, supported by the wall, then still pushing my back further up the wall, bring my legs forward, so that the mere weight of my lower body would gradually stretch my spine into such a position that I could manage to get the glans into my. After a period of practicing this technique on a regular basis I could get my entire penis into my mouth, and eventually I found I could do it from a standard sitting position, without the need for standing on my shoulders. However, as I've already stated, despite it being the wish of so many men, as with many fantasies it's the wish of achieving the unachieveable and, as with many other fantasies, once fulfilled they can prove to be somewhat disappointing & far from the expectations you had of them. As with basic masturbation, while standard masturbation will get the job done, it can't compare to the sensation of having it done to you by someone else. As well as the mental stimulation a partner can provide by simply being there, there is the random aspect. With Mutual Masturbation, as with Oral Sex, you have no control over the pace, pressure & overall approach, whereas when you are alone you have total control & know in advance every following action, thus negating the random factor which is so important to good sex.
Whatever you do, don't strain yourself trying to reach. Just imagine how bad it would look if you broke something - like, say, your neck - and your loved ones found you all contorted, nuts on your chin.