Make sure the second window is really
zoomed out so you can only make out the big outlines, getting this right is the most important thing and makes doing the composition a lot easyer. When i have to do an environment i usually start with a canvas about 200 pix or something like that, get a composition right in black & white, then double the canvas and add color. I iterate this untill i reach the desired resolution and level of detail.
Also make sure to take liberties in what you detail and what not. Usually its best to leave a lot of things to the imagination and only detail parts that are important. I kid of did it in this picture
, which is also quite comparable to the style you're using. As you can see the trees in the background and just outlines, and as they come closer detail is added. This is not because i'm lazy, it's because it directs the eye a lot better.
would be an interesting read as well, it's a study on how to use color in compositions for games to clarify images, using examples from an hypothetical game.