Okay, here's the low-down.
8 Gb Flashdrive & Debian (full); or
4 GB Flashdrive & Debian (mini or net)
I'll use instructions for full.
First off (assuming you're using an Ubuntu based distro) type in:
sudo apt-get install syslinux
this will install syslinux.
sudo su root
mkntfs /dev/sd(x)1 - ('x' is the location of the drive (usually 'b'))
Next, you need to extract some files. Using file-roller, open the Debian ISO (I'm using Lenny). Go to the directory "/install.386" and extract "initrd.gz", "vmlinuz", and the directory "GTK" to the root of your flash drive.
On the root of the drive, and make a new file called "syslinux.cfg". Then add the line:
append initrd=initrd.gz ramdisk_size=10240 root=/dev/rd/0 devfs=mount,dall rw DEBCONF_PRIORITY=medium
Then the final step, just copy your iso to the root of the drive.
That should be it. Hope this helps.