Make a copy of the installer from your Applications folder after it downloads but before you install (it disappears afterwards).
Open the installer package contents by right-clicking and selecting "Show Package Contents"
Drill into Contents -> SharedSupport ->
InstallESD.dmg
Unless it has been modified, the InstallESD.dmg file is the crux of the OS install.
In "Disk Utility", mount the dmg and a USB drive.
Using the "Restore" option, make sure the InstallESD mount is in "Source" and the USB drive is the Destination.
Restore, it will wipe and create a bootable USB drive.
Reboot your Mac holding the "alt/option" key - I use a wired keyboard as bluetooth connectivity during boot can be dodgy
Select the USB install volume
Open "Disk Utility" if you want to erase your HDD first (make sure everything is backed up)
Exit "Disk Utility", install, done.
Hope that helps.
Update: It has been changed a bit. When you mount the InstallESD dmg, inside the "Packages" folder will be numerous install packages. I believe the "OSInstall.pkg" file is the important installer but I am able to use the above instructions without drilling into the Packages folder to restore/create a bootable USB. What's the verdict?