EDIT : JUST SO THERE IS NO MISUNDERSTANDING, THIS IS UNLIKELY TO GIVE A PERFECT WHITE BALANCE, BUT IF YOU GET CAUGHT OUT IN TRICKY LIGHT SITUATIONS, IT WILL GET YOU CLOSE.
There are many add on gadgets for getting an average white balance,
from using a paper coffee filter to the very pricey Expodisc.
Here is another to add to the list, just the top from a cup of Starbucks coffee.
As I usually have a cup or two rolling around in my car, there is usually one I can use in an emergency if I'm out and about :)
I usually keep a few along with straws in the glove compartment too, never know when they will come in handy for a cup of coffee while travelling.
To use, I set the focus to infinity, place the cap 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.