From what I can tell given the information in the dev diaries, articles, and previews it seems to me that Sheperd dying is only a consequence of incompetence.
Such as: if you fail to make relationships with your squad and gain their loyalty, if you ignore the side-quests that can lead to major upgrades on the normandy and on your crews gear, if you don't take advantage of research, etc
If you don't do any of the above, that will lead to your Shepard getting his s*** kicked in and getting killed. Otherwise, there isn't any need for you to "sacrifice" yourself.