Regarding the PSD:
There were more layers than this, but I merged a lot of them together. Still, I try to follow this general strategy:
1) Keep the sketch on the top layer until the last possible minute,
2) Block out the full background in general, flat colors, using the sketch as a guide, and set that as the bottom layer,
3) Put each object (bush, tree, etc) in its own layer group, eg "bush1" is a layer group with all the layers necessary to display a detailed bush,
4) Within layer groups, keep dark colors on their own layers near the bottom, bright colors on their own layers near the top, and midtones (you guessed it) in their own layers near the middle.
5) The first color layers you make are your rough sketches. Subsequent color layers should be progressively more finely detailed, and should on top of the rough layers.
I don't really use adjustment layers. I try to just go straight for the colors I need, and then draw them on the canvas with the appropriate brush.
I used 4 or 5 brushes in this. One constituted ~80% of the picture, another ~10%, and the last three ~10%.