For booting from USB memory stick flash drives, I do it one of two ways:

1. Install Ubuntu in the usual way, but with the flash drive as the target to be installed to. I turn off swap and set filesystems to noatime option to minimize write activities. I currently have a 16GB SDHC card set up like this for my Asus EeePC 900 as a backup boot.

2. Put the ISO image on the flash drive, with modifications to make it bootable as a hard drive (change bootloader to GRUB1 modified with a built-in config file to look for a kernel and initrd at the end of the disk, and append the kernel and initrd extracted from the ISO at the end of the image). Casper (not modified) just looks for a device with the right filesystem, whether that it a CD/DVD drive, or a hard drive. I have 4 of these I use to work on my servers. I have one running on a System76 Jackal Pro 2U right now. You may have to change the boot order of hard drives in BIOS.