I recently learned about the [`fuser`](http://www.google.com/search?q=fuser+linux+man+page) command, which may be useful when I want to unmount a file system that is in use ([`lsof`](http://en.wikipedia.org/wiki/Lsof) is also handy):
fuser -mu /media/USBFLASH # list pids and usernames with files open
fuser -muki /media/USBFLASH # kill, with a prompt, pids with open files
However, a lazy unmount is often sufficient:
umount -l /media/USBFLASH