I've put bits of this stuff on my rims. Just 2 inch strips of it between the spokes in about four or five equally spaced out parts on each rim. Works well with the "box" rims, giving multiple flat reflective surfaces. I have rim brakes, so this only goes on the bits between the spokes to each side-wall.

Other people told me it was pretty visible, but I didn't realize how well it worked until I did the same to my wife's bike and saw for myself. Putting some of the same on the cranks is probably a good idea too. Good spinning parts that flash and catch light back to driver.