I've been marinating for a while on an idea that occurred to me a while back. It's not fully formed in my head, but I thought I'd share it because it has helped me. The idea is that art is a balance between rhythm and variety. In music this is easy to explain - the underlying beat is the rhythm, and on top of that is the variety - long notes, short notes; hi, low; different kinds of sounds. In visual art the rhythm is the repeated bits - like a color that is repeated, or a kind of edge, a certain value, etc. The variety is all the stuff that's different - big shapes, small shapes; curves, straight lines; colors; etc.
The reason I find this helpful is because when I learned about variety I thought that was IT. But if everything is different, you get chaos. Having some stuff be regular gives the piece structure. Art can range from lots of rhythm and little variety, to the opposite. And there are a million ways to do that.