One of the most important thing is to look at the details in the real world and work on it. More you work on the details, more the scene will be realistic.
I dropped out of school when I was obsessed enough with Max and render engines, spending 12-14 hours a day just testing every area. After some time I started getting contacts via the forums where I had posted my renderings to share with others.
You need to really enjoy what you do. ArchViz is a brilliant career but with many obstacles along the way.
My "secret" tool is Vray2SidedMtl. It's a cool way to create realistic leaves for vegetation and, in my opinion, it's better and also faster for rendering than using translucence map.
For achieving photorealistic results general taste in design is far more important than just a technical knowledge of the software.
The key for photorealistic vegetation, besides correct tools, is understanding how it behaves, what it does, what it smells like.
My way to achieve photorealism in lights and materials is always having good references and working strictly based on them.
Analyze distance to vegetation on your render and real necessity for detailed materials - this way you shorten rendering time.
I always try to tell a story. Could be some elements or a light hitting one spot in the scene.
The main components of a great render are: interesting design, good composition, balanced lighting, realistic materials and right camera angle.
Remember: although optimizing textures size, lights and "general rendering setting" speeds the work, overdoing it, can ruin the end result.
To set lighting in a scene, you need to know some photography basics to understand camera settings and white balance.