I used to ride everywhere in normal clothing, but I noticed that whenever I saw someone wearing high-vis yellow, not only did they stand out from the surroundings, but my mind immediately tagged the object in question as a bicyclist. And then I read about inattentional blindness -- the phenomenon of people completely failing to notice something because they were mentally focused on looking for something else. This is apparently a normal aspect of human perception, and it explains a lot of those "I didn't see them" incidents.
Based on that, I decided to start wearing a high-vis yellow jacket. Surely I can't be the only person out there whose brain associates that color with a bicyclist, and it seems to me that the more quickly someone can make that association, the better the odds that they'll respond appropriately. (Around here, most warning signs and road workers' clothes are orange.)