1. Hi-vis yellow (or is it chartreuse) jersey and/or reflective vest. You'll be surprised how much that color shows up under various road conditions. Bright orange or yellow is probably next best. Red is less effective, since it often blends into shadows and the background, and there's a surprising number of us males out there who are red/green color blind. Hot, fluorescent pink is a surprisingly visible color Avoid jerseys or outer garments that are mostly black, grey, blue, green, or brown.
2. Blinky lights. Yep, even in the daytime that helps you stand out from the background noise. And if you're out around sunrise or twilight, you need to have a full set of lights (white headlight, red tail-light, both easily visible from at least 500 feet away) mounted on the bike and turned on. Lights - not just reflectors. We've lost two riders here in Arkansas in the past year from the simple lack of lights.
3. Roadway positioning. Don't be a "gutter bunny." Position yourself where motorists are going to expect other vehicles to be... Typically in a narrow lane that's going to be somewhere between the right-hand wheel track and the center of the lane. Ride predictably -- in as straight a line as possible -- and don't weave in and out of the gaps between parked cars. Always scan behind you to make sure there's no overtaking traffic to either side before you change lanes or move laterally on the roadway. Get a rear-view mirror, and use it. Don't forget to check over your shoulder as well, regardless of whether you're using a mirror.
You get the same effect in the late afternoon while riding west into the setting sun, btw...
Tom