Use threshold layers to find the true white, black and gray points of your image enabling you to set a better white balance.

CORRECTION:  I know I said in the video to set the point sample size to 3×3.. I take that back though. Set it to “point sample”. This is because often the white point is a one pixel catchlight in an eye surrounded by other grey pixels, so if you leave it at 3×3 you’ll get some funky results!

Before image:

See the original post here.