As you probably know, Apple is not going to provide an official USB install disk for Mountain Lion. However, you can still create one using the following simple procedure.
You’ll need a spare 8Gb USB flash disk (they’re dirt cheap starting at $6 on Amazon) or a SD card (SD cards are cheaper, starting at $5 on Amazon, and you use them with your camera later) and the OS X Mountain Lion Install App purchased ($19.99) through the App Store.
First, prepare the medium by launching the Disk Utility and selecting 1 Partition and Mac OS Extended (Journaled) options.