This may come handy, from the project page:
Mk-boot-usb is a perl script to create multiple-bootable usb sticks (usb keys / usb flash drives). It wipes out an entire usb stick, partitions it, creates file systems on it, installs grub, and installs a minimal linux on it. Mk-boot-usb is meant to speed up and lower the barrier of entry for creating bootable usb sticks. The usb stick will immediately become bootable (using the minimal linux), and more useful distributions can then be installed into other partitions manually simply by (1) copying any Live CD into each partition (2) modifying grub's configuration file.
=> Mk-boot-usb: a Script to Create Multiple-Bootable USB Sticks
Related: How to Create Bootable Linux CD
Another interesting article with security in mind. From the article:
You're probably familiar with the live CD concept -- a fully functional operating system on a CD that can be run on any computer that boots from its optical drive, without affecting the one(s) already installed. In a similar vein, you can set up Linux to run from a USB hard drive drive on any computer that can boot from USB. The live system offers automatic detection and configuration of the display adapter and screen, storage devices, and other peripherals. A bootable USB drive can run a mainstream Linux distribution such as Debian GNU/Linux, and can be secured, personalised, upgraded, and otherwise modified to suit your needs.
=> Running Debian GNU/Linux from an encrypted USB drive