Some great thoughts in this thread. I wear a construction vest and safety yellow high-viz long sleeve shirt under it. Since I ride upright (I have a hybrid), it works well for me. I agree with other posters to have more than one rear light, and at least one steady and one blinking.

And of course, at least one forward-facing white light. Mine is mounted on my handlebars. I want to get a second one for my helmet, to help with curves and corners.

And I feel that my Ortlieb backrollers, with their reflective patches, also really help with visibility.

But mostly the construction vest--it particularly helps commercial drivers instantly recognize me as a person. I feel that it helps encourage drivers to give me more room on the road, as well.