Last year we posted a great tip on making Lion into a bootable disc. The good news is the exact same method works with OS X 10.8 Mountain Lion! Details are inside.
Once you have the Mountain Lion installer downloaded from the Mac App Store (available here), navigate to your Applications folder and right click or ctrl+click “Install OS X Mountain Lion” and select “show package contents.”
Now open the “Contents” folder and then open the “ShareSupport” directory. Inside this directory you will see InstallESD.dmg. This is what you will need to burn to have yourself a bootable Mountain Lion install disc.
Simply fire up Disk Utility (/Applications/Utilities/Disk Utility) and then drag the InstallESD.dmg file onto the Disk Utility sidebar:
Then click burn and insert a blank DVD in your burner and burn! That’s it, you have now a bootable Mountain Lion DVD that you can use to install on any Mac you wish.
If you want to boot from a USB flash drive or some other external drive, follow the same instructions as above but instead of clicking burn, click on the InstallESD.dmg, go to the “Restore” tab and then drag your USB flash drive from the left pane to the “Destination” field and hit Restore.