I first tried sucking myself when I as 13 and was able to successfully lick the tip, but never progressed farther than that. Now I am 30 and I have spent the last two years trying to get back the flexibility that I had when I was younger. I have gained a lot of flexibility as I was more than 12 inches away when I started and now can get the head between my lips.
The method that works best for me is the "saving silverman" method [plow position] (by the way, a terrible movie...the only reason to watch it is for that scene) but I do it on the bed and hook my feet under the footboard and get the leverage to get my penis to my lips. If you aren't that flexible, do what I did when I started out. Lay on the floor with a pillow under your back/shoulders to prevent rugburn, and throw your legs back over your head and against the wall behind you. I did this for quite some time to get my back, neck, and shoulders loosened up to where I could actually make contact again. This will not be quick process, but it needs to be done if you don't already have the flexibility.
What you need to focus on is getting your neck, lower back, and leg muscles stretched enough to get close enough to do it.
A few other methods I've used is to sit on the edge of a chair and lock my hands together under my thighs and pull myself forward and toward my penis. Be careful, you can hurt yourself if you aren't careful. This will stretch out your mid/upper back and shoulders. I've looked at myself in the mirror when doing this and I am amazed at how much I can bend. I am probably not far from being able to make contact standing now.
I also have been soaking in hot water in the bathtub. This helps relax your muscles before you try to do these exercises, and also relieves sore muscles when you are finished. I also found that, if you have enough room, I can position myself between the left and right sides (the ones closed to each other), put my feet together sole to sole, and lift them up over my head and try to put them behind your head. This will work your lower back, hips, and hamstrings. I do this often and got to the point where I can usually throw one leg behind my shoulder and foot behind my neck and keep it there. On occasion, I can reach out and bring my penis to my tongue and lick it.
I am not as skinny as I used to be. I am 6-2 and 175 lbs, but I do have a gut and it has prevented me from getting that last inch or two so I can actually get it in my mouth and truly suck on it. I've used a lot of those abdominal exercies, and they really bulked up those muscles, but I can also pinch a bit of fat and when I am in the plow position I can see that my belly is in the way and does not allow me the best angle to get my penis in my mouth. It is said that the shortest distance between two points is a straight line, and this proves that.
I haven't told too many people of my desire to do this, but those that know have asked why I want to do this. I love to masturbate, but using my hand isn't very creative and i've used a few sex toys but didn't get much enjoyment from them. I guess the reason I do this is because it is something unique. Few people can do this and I do feel a bit of pride that I can do this (more or less) after spending plenty of time and determination doing the exercises to allow it. I see it as quite an accomplishment. I have collected hundreds of pictures from the internet and have purchased several videos. I think the whole thing is very erotic. I know a lot of guys into this are gay, and a few people will accuse anybody of wanting to do this as gay too, but I am straight and love women. However, as most guys are, love their penis and want to enjoy it as much as possible. It's just masturbation, and in my opinion, is the ultimate form of it.