1.  Creating a USB version of a live CD — the boring option
With the current 'hybrid' Debian iso's it couldn't be easier.
THIS WILL WIPE YOUR USB STICK

 — START HERE —

a. Download the iso
Using jigdo is a good option. Downloading a business-card/netinstall iso is another, if you'll have a working internet connection available.

b. Plug in and mount your USB device and  find out the device name of your USB drive.
If it's mounted you can use
#df -h

rootfs                                                   93G   36G   54G  40% /
udev                                                    3.9G     0  3.9G   0% /dev
tmpfs                                                   801M  980K  800M   1% /run
/dev/disk/by-uuid/..-10a350f85687   93G   36G   54G  40% /
tmpfs                                                   5.0M     0  5.0M   0% /run/lock
tmpfs                                                   1.6G   52K  1.6G   1% /tmp
tmpfs                                                   1.6G  816K  1.6G   1% /run/shm
/dev/sda6                                               745G  183G  525G  26% /home
/dev/sdc1                                               2.0G  434M  1.5G  23% /media/XP-KOMKU

or

mount

../dev/sdc1 on /media/XP-KOMKU type vfat (rw,nosuid,nodev,relatime,uid=1000,gid=1000,fmask=0022,dmask=0077,codepage=cp437,iocharset=utf8,shortname=mixed,showexec,utf8,flush,errors=remount-ro,uhelper=udisks)

to list mounted devices. In my case it's an old 2 G usb stick I used to create a Windows XP installation USB device on.

If you prefer a gui tool, start palimpsest (called Disk Utility in gnome)
It will be something akin to sdb1 or sdc1 etc. That means the device name is /dev/sdb or /dev/sdc, respectively. In our case, it's sdc.

c. Unmount but don't detach the device
You don't want anything else writing to it.
 

# umount /dev/sdc1
# df -h
Filesystem                                              Size  Used Avail Use% Mounted on
rootfs                                                   93G   36G   54G  40% /
udev                                                    3.9G     0  3.9G   0% /dev
tmpfs                                                   801M  976K  800M   1% /run
/dev/disk/by-uuid/..-10a350f85687   93G   36G   54G  40% /
tmpfs                                                   5.0M     0  5.0M   0% /run/lock
tmpfs                                                   1.6G   60K  1.6G   1% /tmp
tmpfs                                                   1.6G  820K  1.6G   1% /run/shm
/dev/sda6                                               745G  183G  525G  26% /home

sudo su

And write (assuming that the device name is sdc and you are using the businesscard iso)

cat debian-6.0.3-amd64-businesscard.iso > /dev/sdc

Done.

e. boot from your USB drive and go through the same steps as for a CD.  
Plug it in, then start your computer. Hit F12 (or F10 or del or F2) during the bios start-up to select boot medium. Choose to boot from usb. Older BIOSes can't boot from USB.

Fonte: http://verahill.blogspot.pt/2012/02/installing-debian-on-usb-stick-live-usb.html