Can't download Mountain Lion from app store on a Mac already running Mountain Lion?
I would head over to http://support.apple.com/kb/DL1433 and follow the instructions for making a USB Recovery drive.

Also note if you hose your machine you can boot with Command-R to enter into recovery mode.  Even if you hose your recovery partition, if you have an active Wifi connection, Apple will pull down the OS from the cloud.