The Tips in detail

 "Know the material editor."

Your images are living from the textures as they give "the touch". I can't stress this enough: learn what modifier does what to the material. Read the manuals! A huge collection of map images also pays on the long run.

I consider using many different materials makes your scenes tending to be much more realistic. On the other hand this makes it necessarry to know how to work with multi-sub-materials and for sure increases render-time. Real objects are not textured the same way everywhere. Again: look at the real world!

You might notice from time to time that textures look to repetitive due to the applied map-tiling. Get to know how to avoid this via the extended maps and other modifiers. Also add some procedural maps that rely on noise (ie. noise or smoke materials.) This gives your materials that random aspect desired, but it also needs patience (in terms of finding the right "slot" and amount to apply) and can increase renderingtime significantly.