Here is a very short (but in my opinion very useful) how-to for creating an USB boot device, which enables you to boot Debian from your memory stick.

We are assuming that your USB device is indicated by /dev/sda, and that you are interested in Debian stable (other distributions are untested, but should work).

First unmount your memory stick.

sudo umount /dev/sda

Now download the boot.img.gz, which is necessary to make your USB device bootable:

cd ~
wget ftp://ftp.debian.org/debian/dists/stable/main/installer-i386/current/images/hd-media/boot.img.gz

Extract this image and write it to your USB device:

sudo zcat ~/boot.img.gz > /dev/sda

Now mount the volume to /mnt:

sudo mount /dev/sda /mnt

In this case a Debian Stable net-install image is used. You may also use a business card iso. Nevertheless, you should be sure to use the same version of the ISO-image as the image.tar.gz that was used before. Download the ISO to the USB device:

cd /mnt/
sudo wget http://cdimage.debian.org/debian-cd/3.1_r3/i386/iso-cd/debian-31r3-i386-netinst.iso

Now you can unmount the volume:

cd ~
sudo umount /dev/sda

You can remove the USB device, and plug it in your new system. Of course you should tell your BIOS to boot from USB. There is a copy of this how-to on our company's website