OptimUSB is an application that keeps your USB and memory cards optimized. You just need to run OptimUSB before ejecting your USB drive. Any Mac or Windows temporary files get deleted (.DS_Store, .Trashes, .Spotlight-V100, Thumbs.db, etc.). OptimUSB then automatically ejects your drive. Now, your USB drive is clean, optimized and ready to be removed safely.
Remember that OS X generates temporary files every time you connect your USB drive. Run OptimUSB each time you eject.
When you delete a file from a USB drive on OS X, it is moved to the trash, leaving a blind copy on the USB drive. You could completely empty the trash, but you would lose any other files in the trash as well. That is why deleting items does not help you regain space. With OptimUSB, you can keep your USB drive optimized and regain the actual disk space.