In a word, PRACTICE. My great grandmother had a large flat rock that she had cracked so many walnuts on that it had worn a deep depression in the center. She used it for years and years. We'd still like to find that rock. I've used a hammer and anvil followed by dykes which works pretty good. I really took my time with one and was able to extract the entire meat, in one complete unbroken piece, out of a walnut. It was more of "I wonder if" as it took way too long to be practical. but a smack with the hammer and then a few well placed snips with the dykes will get good chunks of meat in relatively quick time. I've switched from that to a cracker along the lines of
this one
My approach - I've always heard that if they lay around too long, the heat from the decomposing husk will affect the taste. Husk them any way you can - knife, hammer, car, etc. I normally drive over them to break up the husk and this seems like a popular method. Probably want to wash them a bit to get the loose stuff off. Then let them dry/cure for a couple weeks. Dunk them in water and throw out the ones that float. Then use one of the cracking methods above. It takes time and practice. And good gloves as the stain will not wash off.
I've heard some 'professionals' say that the juglone in the walnut leaves/husks will mess with other plant life (and worms), but I've heard other 'professionals' say that people always say that but there's no proof.
Keith