Quick summary:

1. Download the installer from the App Store
2. Goto Applications & right Click on Install OSX…. , select view Package Contents
3. Under Shared support select InstallESD.dmg, hold the option key & drag it to the desktop (this will make a copy)
4. use disk utility to format an 8GB USB stick as 1 partition, Mac OX Extended (Journaled) with a GUID Partition table.
5. Then restore InstallESD.dmg to the stick.

Here is the article:
http://subrosasoft.com/blog/guide/create-a-bootable-mac-os-x-10-8-mountain-lion-installer-flash-drive