Tu vs. Vous

This is my current french learning woe: the difference between tu and vous. While in theory I know the difference; "vous" is polite and/or plural and "tu" is familiar, in practice I keep on using the wrong one with the wrong person. The other night on my way home from the gym I ran into a friend of mine and asked her if worked in building she had just come out of, of course I used the wrong pronoun, which basically in France-land is like saying "we are NOT friends". Then during my volunteering stint I used "tu" with a stranger and then made it even worse by excusing myself, fortunately, he was drunk and didn't seem to notice. And then there was the time that Marc's aunt called and I, thinking it was a prospective employer with the same first name, used the polite form until about halfway through the conversation when I realized I was not being hired. Wonder why?