How to install Windows 8 from USB flash drive?
The Windows 8 installer is very similar internally to the Windows 7 installer. All of the other methods mentioned in the previous Super User post on How to Install Windows 7 from a USB Drive should work perfectly with the Windows 8 installer.
I would recommend using the Windows 7 USB Download Tool, as it's a very easy process — you just select the ISO, choose the flash drive, and it automatically formats it, copies everything over, and configures the flash drive to be bootable. I used it to successfully copy the Windows 8 installer onto a flash drive, and the installation went flawlessly on multiple machines.
(Note that when using the Windows 7 USB Download Tool, your flash drive will be automatically reformatted, so make sure to back up any files you have saved on your flash drive before starting this process.)