I had some leftover retro-reflective tape from when I outfitted my daughter's stroller, so I cut 4 pieces of it and placed it on the opposing side of the rim, like this, but not all the way around:
As a driver I find that reflectors moving up and down attract more attention than a fully-reflective sidewall (although that's pretty good as well). Plus I don't really want to be a moving light show, that is actually quite distracting. You can get all kinds of retro-reflective tape off Amazon, in all kinds of colors, including gray/black.