How to Do a Clean Install of OS X Mountain Lion:

Now that OS X Mountain Lion is out, many users are interested in updating their systems with the latest and greatest big cat.
As with OS X Lion, the standard way to install Mountain Lion is via the Mac App Store upgrade.
For most users, this is fine. Some users, however, view a new OS release as an opportunity to perform a clean install. It's possible to do a clean install of Mountain Lion, but the process takes a little bit of work.

Fortunately, we've put together a helpful guide to get you going with a DVD or bootable USB version of the Mountain Lion installer. Does this guide looks familiar? That's because it's nearly identical to the clean install guide for Lion we wrote last year.
The process is virtually the same — however, the DMG file needs to be mounted before restoring to a USB stick in Disk Utility.

1. Download Mountain Lion From the Mac App Store

Download OS X Mountain Lion from the Mac App Store. This download is over 4GB, so if your Internet connection isn't super fast, plan to get some other stuff done while you wait.
As soon as the download is complete, exit out of the installation prompt. We don't want to do an install right now.
2. Find The Install Mountain Lion App in Finder

Locate the Lion installation program in your Applications folder.
3. Right-Click on the Installer, Select "Show Package Contents"

Right-click or control-click on the program icon and select "Show Package Contents." Now, you want to browse to the "Shared Support" folder inside of "Contents" and locate a file called "InstallESD.dmg." This is the file we will use to create our bootable Mountain Lion image.
Copy this file to your desktop or another folder you have easy access to.
4. Start Disk Utility

Open up Disk Utility (it's in the Utilities folder in Applications). At this point, you should see the various hard drives connected to your Mac. If you haven't already plugged in an empty USB thumb drive or a hard drive with an empty partition, do that now.
Select "InstallESD.dmg" in the sidebar. Click open. This mounts the installer image we just copied to the desktop.
The mounted volume on that disk is called "Mac OS X Instal ESD."
5a. Insert a Blank DVD

For users that want to create a DVD for installation purposes, go ahead and put it in your Mac.
In Disk Utility, click the "Burn" icon and then select the "installESD.dmg" file that we just copied to our desktop.
Let it burn.
5b. Select the USB Drive You Want to Use

Click on the tab that says "Erase."
If, however, you prefer to create a bootable USB drive, select it now.
Drag the "Mac OS X Install ESD" volume to the source menu.
For destination, choose the partition of the drive you want to use. Be aware that you will erase everything on this partition and replace it with OS X Lion, so be sure you don't have anything important on that drive or partition.
Click Restore.