Start with Simplicity
When modeling, try to maintain a simple shape and focus on proper curve lines. Cut out the details only when the main shape is established.
Use Fresnel Shaders
When you are creating a car material, you have to keep in mind that, real car material paint is a combination of a few color layers. Always try to use complex colors and include multiple basic shaders. And when you are doing so, try to use the freshnel (it is a frequent angle). It'll enable you to blend between the shades and at a specific angle. You can also implement a specific type of noise algorithm which is called procedural Voronoi and what it'll do is, it'll compute 1D, 3D, 4D Voronoi noises and returns the real locations of the nearest points. Or you can also add a similar texture to normal shader channel to create metallic shade/flakes effect.
Choose HDRI wisely
HDRI (High Dynamic Range Imaging) can be used in order to sharpen and accentuate the shape of the car. My best pick would be High contrast HDRI. What it'll do is, increasing the color punch and enhancing the reflection among the curvature of the car. Always try to use the 32-bit HDRI to get the right color information for your sense. For this task, the simple studio HDRI is worth doing the best job for the most car scenes.
If the environment is not looking as realistic as you want despite using HDRI, then use additional light sources in the scene. But make sure to implement that much of light that will make sure it'll emphasize the shape of your car. And then set up a rim light which will pop out the silhouette of the car.
Use Image based lighting
Using Dome light is really a nice way to add much more color and realistic look to a scene. Dome Light let you allow to adjust the exposure without any clipping. Otherwise, the colored section of an image can seem very black and white-ish, not letting colored data not pop up. It makes both the car and the image look dim and ugly.
You can without much of a stretch tilt your camera to include a sense of movement in powerful and dynamic shots, put a vehicle in an intriguing place with a pleasant color palette. While making a whole scene try to isolate first, the second and third plane, and lead the viewer's eye with focal points & guidelines.
Slope any edge
Don't ever forget to slope any edges in the time of modeling a car. Even a perfectly sharp edge looked by the naked eye isn't actually perfectly sharp. Especially adding a precise and narrow bevel on a refractive element like light defuser will give it a really realistic and high-quality light reflection.