I just took a class at the Mystic Art Center which covered making a small relief sculpture, making a plaster waste mold, and then creating a cold cast bronze replica.. It was an excellent class and I learned alot, both about mold making and sculpting. It was also alot of fun.
For example, I learned that I was holding my sculpting tools wrong – it should be like a hammer not a pen for the best control and a smoother surface, and that I really should go take some basic classes on sculpting… and probably drawing, composition, color theory, etc. 
I’ve figured most things out on my own or through books, and sometimes my trial and error techniques really aren’t the most efficient.
Books are great but there’s nothing like watching someone demonstrate something and being able to ask questions. Sometimes you’ll learn just one small simple tip that the instructor doesn’t even think much of, and it can really help your work.
Btw, I’d highly recommend any classes by Chris Long (sculpting) or Susan Stone(drawing) if you’re in the Rhode Island or Connecticut area.
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You are absolutely right. I took some ceramics classes here in Ann Arbor and it has been dead helpful. Books are ok, I probably checked out every single polymer clay, sculpting and ceramic related books from the library but still you heavily depend on trial and error. Nothing like asking a question and getting an answer.