Great article on making a bootable OSX install image

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