USB flash drives are not indestructible - they may be sturdier than hard drives, but they're still susceptible to physical damage and breakage from falls, electrostatic discharge and data corruption if a device is removed while data is still being copied.
It's good practice to keep multiple backups of your important data, so try USB Flash Copy - a free utility for Windows PCs that automatically takes backups of your flash drive every time you connect it.
No installation is needed and it's only a 300kb download - get it from www.usbflashcopy.com and run it to set a particular folder on your computer's hard drive for backups (by default, it makes a backup folder in 'My Documents' ).
The first time you connect a USB drive, it will take a complete backup of all contents. Thereafter, it only makes small changes to the backup folder - copying only the files that have been changed or added. If you like, you can alter the settings to keep older versions of files too.
And if multiple people use the same PC or if you have multiple flash drives, you can create profiles for each device (with different settings).