To reduce the risk that someone might salvage embarrassing data from your USB flash drive, download a free copy of Roadkil's to match your operating system (Roadkil last updated the software in 2009); then run it while you have the USB flash drive plugged into your PC. Choose the appropriate disk from the drop-down menu, specify whether to fill the drive with random data or just wipe the flash drive to turn it into a blank disk, select how many passes you want the program to perform (stick with 3 to be sure), and let the software go to work.

To guarantee your data's privacy, you pretty much have to take a sledgehammer or a blowtorch to your old drive. But if that's not an option, these secure wipe approaches are the next best thing.