If you've got a cheap USB key lying around which is too small to be useful, follow these instructions to create a rescue "stick" in case your boot loader stops working.

(This is based on these instructions but simplified and updated for grub 2.)

Before you start, make sure you've got both the parted and the grub-pc packages installed.

Make the USB stick bootable

Plug in your USB stick and replace the device name and capacity with the appropriate values. (You can find the total capacity of the stick by typing print inside parted.)

$ sudo parted /dev/sda
(parted) rm 1
(parted) mkpart primary 1MB 4009MB
(parted) mkfs 1 fat32
(parted) toggle 1 boot
(parted) quit

Install grub

Again, substitute sda with the correct device name.

mount /dev/sda /mnt

mkdir -p /mnt/boot/grub
cp /usr/lib/grub/i386-pc/* /mnt/boot/grub/
echo '(hd0) /dev/sda' > /mnt/boot/grub/device.map
sudo grub-install --root-directory=/mnt /dev/sda

umount /mnt

That's it, now all you have to do is reboot with the stick plugged in and test booting your box manually with it.