Wash and immediately dry your copper items after use to prevent spotting! When not dried immediately, spotting can occur due to the trace amounts of chemicals in your tap water that can cause a reaction with the copper. Some people don't mind this spotting; they will actually let it build up on the cup over time into a matte patina finish that many folks find appealing. If you prefer the original bright shine from when you first got your Sertodo Copper hand hammered products, don't worry! You can easily return them to that original condition with a quick polish.
Here is a video describing the basic technique:
And here we have another video presentation about how to return a spotted or patinaed cup to it's original shine: