How to Create an Ubuntu Live USB Persistent Flash Drive from a running Live CD: In the following segment I show you how I used the Official Startup Disk Creator (created by the Ubuntu team) to put Ubuntu on a USB Flash Drive. Ubuntu's Casper Persistent feature can also be used for saving and restoring changes on subsequent boots. Ubuntu is a Linux distribution created by Canonical Ltd

Distribution Home Page: Ubuntu
Minimum Flash Drive Capacity: 1GB
Persistent Feature: Yes
Ubuntu USB Flash Drive creation via CD essentials

Working CD Drive and an Ubuntu Live CD
1GB or larger USB flash drive (I recommend a 4GB if using persistence)

Installing Ubuntu to a Flash Drive via Startup Disk Creator:

Note: Back up ALL data from your Flash Drive before proceeding!

Insert your Ubuntu CD and restart your computer, booting from the Live CD
Insert a 1GB or larger USB flash drive
Navigate to System > Administration > Startup Disk Creator:
Next, (1) Select your Flash Drive from Disk to use (2) Choose to Erase Disk (Make sure you have backed up any important data first)
Now, (1) Select the partition related to your Flash Drive, (2) For Persistence, select the option Stored in reserved extra space and adjust the slider to desired capacity (4) Click the Make Startup Disk button:
A bar appears to indicate the progress of the install. Once the installation is complete, remove the CD, restart your computer and set your Boot Menu or System BIOS to boot from your USB device.

You should now be booting from your Ubuntu Startup Disk