I used my leftover rim tape to add some reflective stripes to my helmet. The reflectiveness doesn't show up as well as in
the pic of my wheels, but it's the same stuff.
Because of the random black patterns on it, the black stripes aren't all that noticeable when they're not reflecting. I just cut some pieces to length and followed natural lines and contours of the helmet.
You can also see a bit of that Purkinje effect (or whatever) in the first pic, causing the helmet to look brighter in low-light conditions. In the pic with the flash, the helmet just looks bright yellow. In the dusky pic, it's got a bit of that unnatural hi-vis glow to it (compare it to how dark the grass looks).