When i used to bike to the university every day, i put silver retro-reflective tape on the inside of the wheels where the spokes are for about 1/3 the circumference. This made it very visible because of the motion and was useful even when broadside because the wheels have a curved profile. I also had tape on the outside of the cranks and on the helmet. Best part was all of this tape was nearly weightless and disappeared during the daytime.