How to create custom Mountain Lion Installer
You could do a clean install of OSX on a drive, install your base software, and set up any user accounts you use by standard (or just one "admin" account). Then use a tool like Disk Utility to copy that install over to a USB drive as a DMG file (in disk utility you use the backup/restore tool).

Or you can just install the OS on that thumb drive and use Carbon Copy Cloner to clone it to any systems you need it on.

Naturally you would need to follow all the terms of the EULA and not install this on anyone's machine who asks.