Restoring a USB stick to its original state:

When you have used the .img image to create an USB installer,
your USB stick is no longer useful for anything else.  Any remaining
space on the stick (supposing you used a larger-than the .img file)
is inaccessible. Fortunately, it is easy to re-create a FAT partition
on the stick so that the USB stick again becomes available for carrying
around your data.

Take care about which device actually is your USB stick. 
The next command will render all data on /dev/sda inaccessible.

First, wipe the bootsector of the USB stick:

dd if=/dev/zero of=/dev/sda bs=512 count=1

Then, create a new FAT32 partition on the stick and write a FAT32 
filesystem on it (vfat or type b in fdisk terminology):

fdisk /dev/sda <<EOF
n
p
1


t
b
w
EOF

mkdosfs -F32 /dev/sda1 

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Original Source:
http://riksun.riken.go.jp/pub/pub/Linux/slackware/slackware-current/usb-and-pxe-installers/usb.txt