I'm going to suggest a specific type of practice, one that I did a few years ago when they introduced the sculpting tools for displacements, but you might want to do for both that and paint mode (assuming you're relatively new to hammer, pardon me if this isn't the case).
Make some arbitrary assortment of connected displacements. Flat surfaces next to walls, inside corners, outside corners, whatever, any size. Then just go through all the tools toying with every setting and seeing what all you can do. The most important part of any aspect of mapping (or, well, anything) is knowing what tools are at your disposal and how they work.
Thinking "I can make my displacements better by doing X" doesn't help if you're not sure how to perform X. Getting comfortable with tools will help you get your ideas out of your head and into the editor and make things that look nicer.