I always shoot in RAW format, but i saw in one Forum a question "How i can correct my white balance in jpg files.
So i decide to write here one quick WB correction method.
First open the picture in PhotoShop , for this method i will use one of my pictures with white plate.
In layers palette
click on Create new fill or adjustment layer
button and select Levels
With right Eyedropper click on the plate (because it's white), probably the picture will gone overexposed, don't worry about it. With double click on the eyedropper you can set the lightness but you don't need it right now.
Set blending options of Levels layer
Sometimes this method is not very useful but 2-3 clicks with eyedropper and everything is going right have to be.
Sorry for my english. I will write more photoshop processing, just say what you need.