I do it in all sorts of positions but when I want to make it an 'experience' .. i make sure that I am in a very comfortable physical position. Usually on my knees or sitting on my heels with his lap positioned where I could rest my head comfortably on it.
If your neck and shoulders are sore, you are probably using them more than necessary. Its good to mix it up... not just for his pleasure and your own entertainment but also so that you are not doing repeated motions that make your body sore.
Alternate hand and mouth and any combination of the two. Lift your head and engage his eyes (a good time for a neck break) while you stroke him.
Sticking your tongue out of your mouth as far as it can comfortably go while his penis is resting on top of it... will push it deep into your mouth without obstructing on the back of your tongue. The more you stick your tongue out... the quicker you can bypass your gag. Oh you'll still gag a bit sometimes ...but no where near like you do when your tongues in the way and its taking a long time to get it deeper.
He's your captivated audience when you are down there. Feel free to take breaks and lift your head, play with yourself or your breasts and let him enjoy the whole show, not only is it appealing to him it keeps you from just bending at the neck repeately getting sore.
Save that energy for when he's close and needs that kind of repetitive consistancy to orgasm

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