1)Never use jet black. It does not exist in the real world

2)Try to avoid using filters. Doing everything by hand will make it look that much better

3)Nothing in the world is perfectly straight in the real world

4)Weather your textures for greater realism. In the real world it is pretty rare to find perfectly clean surfaces.
e.g.Add dents, marks and dirt to them.

5)Always consider where light is coming from and what affect it would produce on the surface being painted.

6)Use complimentary colors to achieve depth and more life-like lighting.

7)Start by blocking out the mid range hues at large scale and get progressively smaller.
e.g.if drawing a round surface like a rubber ball; draw the color you want the ball to be when under strictly diffuse light. Use that color to block in the entire ball.

8)When adding the finishing touches; use localized blur and sharpen judiciously to enhance the perception of different materials in the texture.
e.g.for a smooth surface add a little blur around highlights

9)Similarly, noise can be your friend; use it on materials such as dirt/sand

10)Porus materials can be discoloured. If a heavily coloured material is found adjacent to a porus one, it is likely that the later will be stained by the former. For example, paint bleeding into wet/damp plaster.

11)Use layers!

If you're making scratches on metal.
Use the smudge tool to make it more scratchy because smooth lines are rare as scratches on metal.
My example:

Spoiler tags would be helpful since there's large images. >:/

I just found some "Terragen" program, it can be used to make Skyboxes/Landspaces.

Try 'googling' it.

that program is wicked, especially when rendering with detail all the way up.
Although, you will have to find the center, and the right alignments for it to project like a box.