Read your work aloud! When you read aloud you find out how much can be cut, how much is unnecessary. You hear how the story flows. And nothing teaches you as much about writing dialogue as listening to it.
The bigger the issue, the smaller you write. Remember that. You don't write about the horrors of war. You write about a kid's burnt socks lying on the road. You pick the smallest manageable part of the big thing, and you work off the resonance.