Its one of the main rules of good painting and I have to admit its something that I don't do and should.
Most of the time I just take the paint from the reservoir in the lid after a good shake up.
This post is really a plea for help, I'm looking for peoples advice on what to do with paint thinning, do you just use water or some sort of agent? Do you water them down on your palette or in a mixing pot? How many more coats do you find you need to use once watered down?