Skin that looks “plastic” typically results from many small decisions compounding together. If you heal just a little too much, blur some texture to hide it, push a bright spot on the face up a few stops, and then do a heavy touchup pass, you remove natural detail that the brain uses to tell “reality” from “edit,” and you may not even notice what is happening. But it is likely not “real.” The image no longer has depth or detail, no texture, no pores and just a little too soft for it to be a face.
The best defense against plastic skin is to not mix cleanup and smooth. Cleanup is for temporary things: things that shouldn’t be there, like acne blemishes and hair, dust spots, and marks on things that shouldn’t be in the photo or even close to the face on the subject (or just behind the face in a very light, out-of-focus area). Smoothing is for smoothing out areas that are big enough to notice the difference in tone and texture. If you do those things together too aggressively you are using too large of a brush too hard, and your skin will just get worse and worse looking in the process. Use the smallest healing tool and only do what’s needed for cleanup and see if anything beyond that is necessary.
Strength in your healing/smoothness brush is as important to the result as the size of your brush. Keep the strength, and especially opacity and flow, low so the changes happen slowly. You don’t want the entire cheek, chin, and forehead area to be covered by your brush. If you don’t see where your stroke is or if you do and see that a lot of what the brush is touching is already being changed or is being changed by that same spot on the brush stroke, then your tool is too strong for its own good and you should slow it down. If you are smoothing a mask or dodge and burn layer, keep the soft edge, soft brush, low flow and the results will look soft and smooth without being plastic. You also should see what you change is blended into the rest of the face and there isn’t just an island that looks different than the surrounding face.
Look for repetition of texture and lack of texture to help judge your results. Repeated texture is something that happens because of cloning from the same area in a short period too many times or using an area too big to the point it repeats in other places. Lack of texture is caused by the same thing that happens when too much smoothing or healing has been done. You can tell you’ve over-done it because it’s a uniform patch and looks nothing like the rest of the skin in the face. If you don’t see the result then your work has been done in a way that you haven’t seen enough of what you did to see, or you have overcorrected or under-corrected it. You don’t need to identify exactly what is wrong if the area looks out of place compared to the rest of the face. If you have pores in one area and nothing in another area next to it that should have, it might just look smooth or even, but it won’t look right to anyone’s face.
Pushing the saturation can create artificial looking results as well. If there are reddish, orange or purplish tones that are too strong they will look like paint, especially on the face, or other places if they are in the area being retouched. Darkening the highlights and/or brightening the mid tones and shadows will lose the edges of the face in those areas, especially around the eyes, nose, chin and cheeks. Don’t be in such a hurry to go back and do another cleanup pass that you skip looking to see if this color or light issue is causing you the most problems. This is a time to be sure about white balance, color cast, shadows and highlights before deciding you need to heal or smooth something else. You don’t need to add another layer just to smooth the surface of it, either. Sometimes the area that is needing retouching is needing some small adjustment that is better done with less of a tool.
A good exercise to practice this technique is to find a portrait that has some things that look out of place and do nothing but clean up one side of the face. Create a layer for the clean up and do the most visible things to see what it would like if that was done. Don’t do pores because you shouldn’t see them and if you can tell you have cleaned everything up and the skin looks just like it was before then you did a bad job. Now do a tone correction of that area of the face, again on a separate layer, so you aren’t changing the rest of the face if it needs it. Use as little opacity as you can see a little bit of change and that will be enough. Look at your image, turn the clean up layer off and on. Then turn that and the tone adjustment layer off and on so that it only changes if the layers are both visible. See if you made it better or just different.
Don’t do so much that it looks like a perfect skin. A real face has skin and real skin has spots, pores and small dark shadows or patches. A lot of retouching will make things worse, not better because it will only cover things, not improve them, so be careful. A face looks natural when it looks real and not when it looks like nothing is wrong with it and nothing can be done about it.