How to build good jumps?
You could probably rent a Wacker ( yes, that is the real name ) at a local construction equipment rental store. It is a tool that ground construction workers use. It is basically a really heavy steel plate that an engine sits on. The engine vibrates the whole machine and the weight and the vibration compact the ground very tightly. 

You'd have to work in layers. Don't expect to compact the whole mound all at once. That is not the way it works. 

The giant rollers on freeway construction sites do the same thing.