While using USB drive on computer, some files are automatically copied on the drive. These may include files like thumbs.db, .Trashes folder, .Spotlight-V100 or the ones with the underscore at the start of the name.
USB Drive Fresher is a free utility to delete and clean up USB drive from such unwanted files. It support lot of customizable and predefined cleanup patterns. You can also perform autocleanup of USB drive when it is plugged to the computer.