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Ubuntu comes with a startup disk creator ideal for creating live USB jump drives to boot from. The Ubuntu team designed it so that it can make USB startup disks for almost every distribution, so even if you are not wanting to boot Ubuntu you can still use this program.
In order to run the disk creator, just type "sudo usb-creator" into your terminal or navigate to "System -> Administration -> USB Startup Disk Creator". Once you have opened the disk creator you can select the disk image to use by clicking "Other" and then select your USB drive. Finally you will want to decide whether to make it a persistent or live installation, a persistent installation is one where things such as your documents and settings are saved. You will want to use persistent if you are planning to boot from USB for a long time. If you are just planning an installation then live should be fine.