Remember "Sky of Fire"?

If you do, then you probably remember that I was writing it for Camp NaNoWriMo this year. As it turns out, I didn't win, but I did manage to write 18000 words of it. Considering it's an action-oriented (possibly science fiction) work, I'm aiming for around 70000 words.

With that being said, I do have a slight issue that I'm running into; I'm failing to describe things well enough for my readers. The two people I have proofreading it say I'm not giving enough description of the surroundings, and focusing more on the characters. While that's something I love to do, I need to fix that aspect of my writing before I publish anything.

So are there any tricks you guys know of to help me flesh out the surroundings without going too purple?

~SoI