Getting accurate white balance can be tough, especially when you have mixed light pouring into a scene. If you don't have professional white balance filter (like the Expodisc), you can still do pretty well with a regular old takeout coffee lid.
Photographer Steve Bennet discovered the lid—while not flawless—performed remarkably well as a white balancing tool. How does it work? Much like a proper white balance filter:
To use, I set the focus to infinity, place the [coffee lid]over, then set the custom WB. I dont even need to hold it as it fits nicely inside my lens hood, but that is just a lucky coincidence.
Some argue a bleached coffee filter is better, but that may be harder to come by in a pinch, while coffee is never far away.