Use reference pictures to get an idea of how different materials should look under different light conditions and the way highlights/shadows naturally fall.
Like if you are painting a musclebound brute, look at some body building pics.
If you are painting shining armor, take a few pictures of a suit at a museum or at a larp/cosplay/re-enactment meeting.
The other thing I've found most effective is watching videos and paying particular attention to how the artist holds/uses their brush and the different way they manipulate/transfer paint with it.
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