Know what you draw or paint

One of the first things to learn in drawing or painting, like being a writer, is to know your subject matter. You can't write about something you know little about. Similarly, you can't paint well what you don't know, even when you can see it. The eye plays tricks and knowledge of the subject matter corrects this. If you have a background in a particular subject, you have knowledge of that subject matter. Paint it. If you are used to being in forests or around trees, your paintings will have more value if you paint trees. Same goes for houses. If you have a background in architecture, for example, paint or draw buildings and houses. In short you will do your best work when you have a background in whatever you paint.

That is why people who paint portraits try to get to know their client before painting. Knowledge of subject matter is key.