There are many, many people out there which are rediculous at photoshop, but there are very, very few who can use photoshop with eloquence. A LOT of what I learned was to just sign up on forums and whenever someone post up a picture just ask them what they used to change it from the original and play around with it for a few days.
I lot of the time you can just find some good tutorials all over the web.
http://www.pixel2life.com/search/4/photography/1/
http://www.pixel2life.com/search/4/color%20balance/1/
It is just repetition really. Photoshop in my opinion isn't as much knowing right away off the top of your head what you will do and how you will do it, it is knowing what you want to do and knowing how and where to get the resources for it. There are so many things you can do in photoshop it would be absurd for someone to memorize every single thing.