The answer is fairly easy, yet most people don't see it: practice.
If you spent the same amount of hours drawing backgrounds that you use doing characters, your backgrounds would look much better, but backgrounds are usually seen as "homework", and only few artists don't skip them.
I started using photos of mountains and natural landscapes, adding a few buildings here and there, and piece by piece, creating proper backgrounds. My advise would be don't get lost in the details, which is easy to do when you're drawing digitally. In comics, you just need to suggest what's in the background, it's not an illustration.
When it's a contemporary city i use lots of photos and merge a couple of ideas into one panel/scenario, like a collage.
Sometimes i also use 3D objects, mostly for sofas and similar, to save me time and change the scene's perspective. Creating the background first is always better.
Oh, and i like to add clouds on forced perspective on the sky, they're super helpful. But that's me. I like drawing clouds.
