I have come to believe there are no little things. I have been fascinated for a long time by the Sliding Doors phenomenon, from the movie of the same name. Miss a subway train by a few seconds; change your life.
This way lies madness of course, because you can second-guess every move you make. Still, some moments seem to be obvious candidates. Minute decisions (sometimes by you, sometimes by others) with monumental consequences. It does really seem sometimes that the impact of most conscious decisions is inversely proportional to the time and effort that goes into making them.