The LED headlamp harness (yoke) failure thread reminded me of this, and I figured maybe someone would benefit from my "solution".
I have found that Princeton Tec Quad and Eos headlamps tend to fall off their elastic bands very easily. The simple solution seems to be to put the elastic through from the "wrong" side. If you look at the pictures here, you'll see that the elastic enters between the headlamp and the pastic "backplate". The picture doesn't show it, but it then goes behind the backplate and then back in front of it.
If instead you put the elastic in behind the backplate, and have it go in front, and then back behind, then it seems to hold a lot better.
I'm sorry I don't have a camera with which to show this.