How does one break long bones for broth? When attempting to cook bone broths, what is the correct technique of breaking up long bones which do not fit inside the crock pot? 

I can think of hammering or sawing off hand, but both seem to be rather messy techniques. Is there any good method to break off large bones (relatively) safely?
A normal hammer (one that is used for nails) will not suffice for breaking relatively strong bones, such as the leg bones. 
Personal experience: I bought a pork knuckle, and attempted to break the bone with a hammer (600g head), and it resisted 5 minutes of straight hammering, despite the fact that the surface was significantly dented, and many bone fragments were scattered about the room. 

A few online searches suggested that a sledgehammer would be required to break bones for broth. 

In short, get the butcher to do it for you. You probably don't have the necessary equipment needed to smash bones.