I recently learned about the fuser command, which may be useful when I want to unmount a file system that is in use (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