-using not too many black outlines next to each other, especially not if it's really crowded.
-By introducing a small palette, which has colors which contrast really well with each other you will have a much tighter control.
-use only one black color for all of those dark tones.
-if you have one black and good conmtrasting colors you can only set an outline on one side (like in the wooden framework in the edited house wall) - which then will add a lot of dimension to some details.
-Get rid of texture where it hurts the piece and flatten the colors, like on the ground.
-If you want to use texture with that few colors, use bigger areas of the same colors, connecting together in massive blobs of colors. All those tiny details take a long time to draw, but aren't really as impressive, as solid basic forms.
Take my edits of the piece as a suggestion to underline the points i mentioned above.