Last year, I posted about creating a standalone Lion USB installer thumb drive. This year, I update those instructions for Mountain Lion.
You'll need a copy of the OS X Mountain Lion installer. If you saved a copy when you first installed Mountain Lion, great. If not, you'll need to re-download it from the Mac App Store.
Launch the App Store and option-click the Purchases tab. An "Install" button should appear next to Mountain Lion. Click it to re-download the installer. You can use this option-click-Purchases trick to re-download any purchase, not just Mountain Lion.
Once the large multi-GB installer finishes downloading, go to your Applications folder to find the installer itself. It is called Install Mac OS X Mountain Lion.
1. Right-click (or Control-click) the installer and choose Show Package Contents from the contextual pop-up. A new Finder browser window opens, showing the normally hidden material inside the installer bundle.
2. Navigate to Contents > SharedSupport. There you'll find a disk image called InstallESD.dmg.
3. Open a new Finder window with Command-N (File > New Finder Window). Navigate to /Applications/Utilities and launch DiskUtility.
4. Attach a thumb drive to your Mac that is at least 8GB in size. Prepare it for use by creating a single HFS+ partition. Select the drive (e.g. SanDisk Ultra) in the left hand column. Drives are listed first with their partitions listed after them, each partition indented slightly.
5. With this drive selected, click the Partition tab and choose Partition Layout > 1 Partition. Choose Format > Mac OS Extended (Journaled).
6. Click the Options button at the bottom-right of the partition layout. Select GUID Partition Table.
7. Click Apply. Disk Utility asks you to confirm. Click Partition. Wait as it unmounts, partitions, and remounts your disk.
8. Next, select the new partition (Untitled 1 by default). Click the Restore tab.
9. Click Install next to the source field.
Drag InstallESD.dmg into the file-open window and click Open. Mount InstallESD.dmg (by double-clicking) and drag the mounted image from your desktop into the source location (Thanks, shamguy4) Drag Untitled 1 from the left column to the destination field. Click Restore and agree to Erase the drive and replace it with the contents of InstallESD.dmg. You may have to authenticate as an administrator.
10. Wait. It will take some time for the drive to be written. Once it's done, eject it, label it clearly, and either use it to install Mountain Lion on a newly formatted hard drive or if you've already upgraded through the normal installer, put it away for a rainy day.
Those of you who want a more turn-key solution should consider Lion DiskMaker, a donationware AppleScript app that will let you make a bootable DVD or thumb drive from a Lion or Mountain Lion installation program.