Shoot/no shoot

After some practice and though on the matter it seems to me that the longest part of your reaction to an attack or possible impending attack is the mental recognition of what is taking place. I can practice rapid presentation and firing from concealment as much as i want BUT how do you become more proficient in making shoot/no shoot decisions, and how intrusive do you allow certain "just to be safe" type techniques to be in your day to day activities (ie crossing the street if a group of shady looking characters is walking up the road towards you, not getting on an elevator with people, etc) that allow you more time to make these decisions should the need arise?