To set lighting in a scene, you need to know some photography basics to understand camera settings and white balance.
Be self-critical with your work: try to find the balance between the time spent on a project and the quality.
I prefer to spend less time in 3d-software and more in postproduction. For me Photoshop is the best render engine.
Start with a small scene and shoot it like a photographer, remember the composition - it considerably changes the image perception.
Photoimages serve as the basement for fixing materials and lights. Results in 3d strongly depend on the choice of references.
Light is the most important in CG, above modeling and texturing; it gives the render right emotions, even without textures.
You need to spend time analyzing the real world to be able to imitate it. Inspiration comes from the outside.
To obtain photorealism, remember: everything in this world has a certain imperfection: color, reflection, geometry, placement or all of them.
Anyone can learn the software, but to be outstanding 3d-artist, I suggest to gain some sort of visual communication literacy.
With time we'll get even closer to reality, it will be hard to distinguish a virtual image from a photograph.
3D is just a tool, so the most important is to provide the solution for the needs of our customers.
The difficult thing is to keep up with growing visualization technology. You have to keep learning to be on top.