Frequency separation is a powerful way to edit portraits, but it's often misunderstood or misused. Nevertheless, when done correctly, it's a quick and effective method. This helpful video will show you exactly what frequency separation is and how to use it on your images.
Frequency separation is hotly debated in the retouching world, but I personally think it's perfectly fine when used properly. The idea is simple and arises from a mathematical concept called the Fourier Transform: intricate data such as pore texture is carried by higher frequencies, while more broad data such as color and lighting is carried by lower frequencies. If you separate the higher frequencies from the lower frequencies, you can work on one without affecting the other. This awesome tutorial from PiXimperfect will give you a bit more background on the theory and technique and will show you exactly how to use it on your own photos. Just remember that less is more and to slowly build up the effect, particularly since it's destructive (on the created layers).
And if you'd like continue learning the basics of photography, be sure to check out "Photography 101: How to Use Your Digital Camera and Edit Photos in Photoshop!"