From a technical standpoint, the three biggest tips I can give you are:
1.) keep as few layers as possible: preferably one. Painting over your mistakes will make your workflow faster.
2.) work with a solid color scheme. If you're going for a mood piece, especially a quick one, having too many conflicting colors will make your work a mess. I always paint the entire picture a base color (green, blue, brown, etc.) in half transparency first, to help unify my palette.
3.) either sketch or block in your picture first. Compositions are difficult, especially in action pieces, and you don't wanna scrap a halfway done piece just because the composition is off.