While this is not a shading tutorial, it’s still worth saying a few things about materials as bad materials can kill the photorealism in your scene. Getting the perfect shader takes a long time and books could be written about them (Grant Warwick’s V-Ray course is a great place to start and you can check my material creation tips here & here). But there are a few simple things you can do that will prevent your materials from damaging your render, and they all have to do with textures and colors.
Essentially, you should be careful about not setting any color values in the diffuse slot too close to the maximum RGB value of 255 whether you’re working with colors or bitmap textures. This applies to the individual red, green and blue channels. Excessive RGB values will tax the renderer, extending render times, and give your image an unrealistic over-saturated look. Likewise, you should never use pure white and pure black in the diffuse as these never occur in real life. Pure black will absorb all light and pure white will bounce all light rays back. As a guide, I use values around 180, 180, 180 for white in all my scenes and never go beyond 200. If you work with textures in the diffuse, color-pick them to make sure they don’t have excessive RGB values. I also often slightly desaturate the textures I make from photos in Photoshop before using them. You may also want to tweak your textures to make sure they all “sit” well together. This is particularly true of saturation. For example, although the trees in the scene below use different leaf textures, some warmth was added to make the whole foliage “sit well” together.