To get a feel for what kind of keywords will make for great Midjourney output, I like to pick an artist name and ask for a 'landscape by <name>’. A really basic prompt — effectively ‘when I say this person's name, what comes to mind?’; makes for an easy comparison point. Oftentimes generated art looks nothing like what that artist would actually create, but still conjures distinctive reusable style elements
Names used here:
1) David Hockney
2) Takashi Murakami
3) Yayoi Kusama
4) Kara Walker