Alright so you have the right environment (check), right posture (check) and now you have to apply strong determination. Similarly to not reacting to figurative lawnmowers of external sounds or disturbances, you need to learn to not react to your body sensations and mind’s stories. Think of this as "don't itch the itch" like knowledge from the little gnome figure passing you wisdom in the game. If a discomfort sensation arises, try not to alleviate it by shifting/itching/getting up. Use it to sharpen your neutrality and concentration as well as wisdom that it is impermanent. There is nothing like watching an itch arise and pass away without intervening. Try it at the next opportunity.

Your mind will also try to convince you to get up. It will do all kinds of things to subvert your progress in the game. See these as the obstacles of the game and smile at them. Give them neutrality and maintain concentration while you await their demonstrated impermanence. 

It is almost as if suffering (pain, itch, discomfort, unease) is needed in order to truly know what impermanence and highly tuned neutrality is. You almost have to know it beyond the self that gripes, complains and desires comfort.