There is simply no secret, instant fix to attaining truly great skin tone in postproduction. Using Photoshop, you can accomplish many a miracle in portrait retouching, but the many variables in any one image will dictate the direction you will end up going in your workflow when you want rich, deep, vibrant skin tones. I made an Action that starts the process for you in a click using my favorite approaches to deepening skin tone.
The thing to remember, however, is that no two sets of images will demand the exact same approach. In all retouching work, you need to dive into an edit with an approach, not a linear set of step-by-step bullet points, and be able to remain flexible and adaptive. After all, every shot is different and every new shot will throw a curveball at you that the previous one didn't.
That said, when I see a situation in a shot that lends itself to enhancing some truly glorious deep skin tones, I run with my favorite process to get things started. Also keep in mind, a lot of your searching for the perfect skin tone starts in raw processing, and quite frankly, you should strongly consider using Capture One Pro.
As you can see in the before and after images, this Action gets you started on deepening your skin tones so you can then continue in your workflow with a great start towards achieving a darker, rich look to skin.
Once in Photoshop, and whether your subject has very dark skin, medium dark skin, or simply a summer tan on fair skin, how you go about deepening the tone is crucial. The first thing that goes wrong, and often does, is accidentally oversaturating the skin color with sloppy darkening techniques. The second thing to go wrong is simply darkening skin tone too far. The proper technical approach and appropriate vision for what looks tasteful and balanced are what will get you that perfect, deep skin tone you've been after.
I decided to make a Photoshop Action that does the first two steps I tend to begin with. You can download this Action totally for free from my website, right now. Watch the video tutorial above to see how I use it, and I would love to hear what you think of it.