As most of you downhillers know, the first part to break on your helmet is the visor.
In my unprofessional opinion the visor isn't really for protection, it's mainly to keep sun out of your eyes so I felt safe epoxying mine back together.
WARNING: This will void your warranty and crash replacement policy.
Step 1: Epoxy
Line up the visor pieces as if it wasn't broken (do this while it is still on the helmet, NOT as shown in the picture). Apply epoxy to the outside first. I recommend plumbers epoxy as it is extremely strong and simple to use.
Don't worry too much about the roughness, you can sand it after it dries.
Wait the appropriate amount of time according to your specific epoxy label.
Remove the visor from the helmet.
Then apply epoxy to the inside of the visor.