It's true. Flash can look artificial. I've even heard photographers say that they can always tell difference between natural light and flash. Well...the truth is light is light. There isn't any difference between natural light and flash. They are, in every way, exactly the same.
Having said that, there are two reasons why a flash can look artificial:
The flash is small.
The flash overpowers the natural light.
Small flashes make harsh shadows. This is one reason why a flash looks artificial. However, this blog's point isn't about how to effectively increase the size of flash - that will be for another post. This blog is about ensuring that you do NOT overpower the natural light.
Overpowering the natural light is a very common reason why flash often looks unnatural - because it's overpowering the existing light. The simple method for ensuring that this doesn't happen is to follow these three steps:
1. Turn your flash off. You need to understand what's happening with the natural light before you introduce the flash. You can only see what's happening with the natural light with your flash OFF.
2. Set your camera so the image is SLIGHTLY underexposed. Flash looks unnatural when it dominates the natural light. Set your camera (f-stop, shutter speed, ISO) so that the image is slightly underexposed with the natural light. Remember, you're about to add light with a flash so you want the natural light image to be slightly underexposed.
3. Turn your flash back on. I set mine to ETTL and then make adjustments from there. Generally, the flash is very good at producing the right amount of light to get correct exposure.
Again, the goal is to make the flash slightly more powerful than the natural light. Even if you're using a small flash, it will look less harsh because the natural light fills in the shadows created by your flash.
It's like most things in life... if you take a second and see what's happening around you, you'll more clearly see what you should do. :)