REUTERS/Shannon Stapleton If you're sitting on any losing investments, December is a good time to take a step back and decide whether you still want them in your portfolio. Thanks to a strategy known as tax-loss harvesting, you could use those losses to help lower your tax bill, and then put that capital to use on more promising investments. Here's how it works and what you need to know. What is tax-loss harvesting? Nobody is right 100% of the time when it comes to investing.