You shouldn't just improve the visibility of your dog, but yourself as well, since you are likely the larger of the two. As suggested by @ratchetfreak, you should get a high visibility vest for your dog, but also consider one for yourself.
Additionally, I have seen people clip reflective objects to their leashes. A "poor man's fix" would be to clip safety pins with aluminum foil to your leash, but if you need to resort to that, you have bigger problems. If you can forgo your manliness, try using a gaudy rhinestone leash (and collar), as those are quite reflective. Or you can simply buy a reflective dog leash.
You have your dog leashed, so you have control of your dog. Since you have control, walk with your dog on the side of you furthest from the street, as you (the larger object, likely with the now-reflective vest and hopefully wearing light-colored clothing) are more visible to cars than your dog. You really shouldn't be walking your dog in the street in the first place, but it is understandable if you have no sidewalk.