Once i saw the title I had an immediate answer. Quite simple:

a) NOISE. You need noise. So much that it is not too loud but becomes background and ignorable. Practice on the NYC subway.

b) Head Rest. Something to keep your head in place and no pressure on the neck. Do that and you can sleep sitting or standing (yes I can)

c) Nobody touching you. anyone around you can do what they want, but as long as body-contact does not CHANGE all is well. You can be touching their hand or shoulder to shoulder, but as long as there is no change you are fine. Sleeping in weird non-super-restful positions = you are more alert so small things will wake you.

Again, practice on NYC subway and you will be an expert in no time.