add imperfections

The perfect cube cannot exist in real life. When you open blender the first thing you are greeted with is the perfect cube. Break it.

most things in life don't have razor sharp edges

Add even a slight bevel to everything to help it catch the light in a more natural manner. This is where its useful to have nice geometry. automatic bevelling tools don't play nice with over-complicated geometry and are more likely to mess up the normals, thus creating lots of weird artifacts if you try to use normal smoothing, if its even able to produce an ok mesh to begin with.

most things in life aren't perfectly flat or smooth

Use a smooth noise texture to slightly muddle a flat surface wither through displacements or wobbling the normals.

most things in life are not perfectly one color

If you have a large object its usually worth slightly muddling its color either with a texture or by applying some bump to its surface.

most things in life are not uniform

add rough spots and divets to surfaces. try to avoid having repeating tiling patterns, the human eye picks up on these really quickly and makes things look artificial. If you have a bunch of instances of the same object add little random variations so they all look subtley different. Maybe warp each of them by a different amount seeded by their position or object ID, maybe give them each a random color variation.

many things are not perfectly opaque or transparent

In the blender default BSDF its often worth taking advantage of the subsurface scattering options to add a little bit of realistic color variation to organic materials like bone or marble. Be sure to consider what the undertone of your material is and what colours of light travel the furthest inside it. By default blender's subsurface scattering is set up to simulate light travelling through the blood and muscle of your flesh.

dirty it up

If you watch moveie-prop makers at work you'll see that a huge part of their work is adding a realistic patina to otherwise perfect objects. In his build videos Adam savage talks a lot about how useful "damage" can be in selling a prop as well as its storytelling.

add wear and tear

Break edges, add dents fleck off bits of paint to expose the metal underneath all of these little signs of damage work together to give an object heft and a sense of materiality. A wooden plank has splinters and plugged knots. An old fire hydrant has rusted bolts worn corners wear the paint has rubbed off and patches where it's chipped away. An old glass in a diner has a line ofscratches where another glass slots into it and fingerprints from the hand that brought it to the table. If it's a cold drink maybe there's beads of condensation and a pool of water at the base.

greeble it

While tech products often try and communicate how powerful they are by the sleekness of their housing an industrial tool might have dozens of wires or hydraulic lines running across its surface rather than a smooth hull. Or maybe a shelf has extra unused mounting points that we can still see. Even when in real life something probably wouldn't have so many little surface parts adding them helps convince the viewer of their authenticity.

dress the set

have you ever walked into an empty house? One where the interior is fully finished but it just doesn't feel lived in? it's because there's not enough stuff. There's too much empty space, too many uninterrupted surfaces. If you have a dining table cover it in magazines and plates and cups. Put posters on empty walls furniture into empty floor space and so on.

put some dust in there

You can use ambient occlusion to push details and simulate dirt and dust getting stuck in cracks. For large objects you might want to add some textures or decals of water and rust damage like you might see gathering around a ridge in a ship hull or a crack in a concrete wall. Adding dust in the air or small insects can also help ground a viwer in the particularities of a scenes environment. For example an abandoned warehous has dust settled over the floor and any remaining boxes while the lone adventurer leaves behind footprints and kicks some dust into the air; or maybe you want a misty fishing dock with bits of ocean spray whipping over the end of the pier.

use photos

The fastest and cheapest way to add realistic weight and color to a scene is to steal it directly from real life!

photobash!

Look how often people like Ian Hubert use photobashing and photoscans to quickly and easily create realistic set dressing for their work. For hero objects you might need to put in a little more work but for something that isn't the focus of a scene quickly cutting out what you need from a photo or using a photo to bash out a model will give you something with enough detail and of the correct proportions that most people's eye will just scan over it without incident.

use HDRIs

Setting up natural looking lighting can be time consuming and harrowing AND you still need to build out a realistic background set. Skip all of that with HDRIs these give you naturalistic global illumination and background detail for free!

pay attention to size

It can be tempting to just sort of eyeball everything as you go without really paying attention to any of the numbers along the way but don't do that!

naturalistic proportion

If you're building a human environment make sure to size things proportional to humans. If a doorway looks a little too narrow or a table too tall a viewer may not conciously notice the problem but they will notice that what they're seeing feels off somehow. In fact many parts of our manufactured environment have standardized sizes and proportions so even a slight deviation might seem weird (you ever go up a stairway with a slightly awkward step height).

naturalistic scale

light looks and behaves differently at different scales. A fish tank of tea looks almost black, whereas a milimeter of tea is almost clear. If your translucent or transparent objects aren't true to real-life scale they might look too dark or too light. If I have a glass cup thats about a meter cubed in dimension it will look weirdly dark since its walls are much thicker than they would be in real life and thus scatter and absorb more light than we have been trained to expect of a cup and thus the cup looks weirdly dark.

make it look like a photo

Most of the naturalistic images we see are produced through the use of a camera, thus the more we make our renders look like they came from a camera the more realistic they will look.

add depth of field

Cameras don't have infinite focal length. In blender you can set a camera's depth of field to keep a specific object in focus. I like to make it follow an empty that I can then move around however I need.

simulate camera flaws

While raytracing is more or less based off of how real-life optics works it doesn't simulate all the visual quirks you'll see using a camera. For bright lights you'll need to add back in lens-flares and bloom otherwise they'll look odd and underwhelming. If you're capturing something in motion you'll need to tell you're engine to simulate motion blure or add it in yourself. You may also find yourself needing to simulate light leaks, chromatic aberrations, or vignettes. These sorts of camera imperfections aren't always needed but every now and then can help ground your point of view and sell the image.

pay attention to your focal length and f-stop

If you're taking a closeup shot you'll want to simulate macro photography with a very narrow depth of field (think f-stop 2.8 or so) and vice versa for wide shots. Maybe you want a telephoto lense to photograph a building maybe you want a wide angle lens for a landscape.

light it like a photo

Learn how photographers think about lighting. Do you want a hard light or a fill light? what color temperature makes sense to use here? do you need a rim light? how about a fill? The lighting in photographs isn't exactly "natural" but we're so used to seeing it that having a render follow its conventions won't just make it look better but more real. It's also worth noting that unlike cg artists photographers are still limited by physical reality and so we should try our based to replicate those same limitations.

check your color management

You might be rendering with too low of a dynamic range or something that skews saturation funny or any other host of things. Digital color is a big complicated field with lots of writing, but all I'm really gonna say is that, if you use Blender you should probably be using the filmic color transform. Andrew Price's video explains it in more detail.