Here is a quick Photoshop tutorial on how to make bad skin look great. Retouching skin is not a simple task. You can easily overdo it and make the skin look unnaturally airbrushed and almost cartoonish. That being said, with some care and a few practice runs you'll be touching up skin in your photographs with Photoshop in no time.
Open your photograph in Photoshop
Right click on your background image in you layers window and click "duplicate layer". You'll now have a clone of your background. This is important for both masking your image and adding the right skin texture later on.
This is where you make most of the changes to your subjects skin. Make sure your second layer is active and go to your "filters' button at the top of the Photoshop window. Click "filters -- Blur -- Surface Blur". Ensure that "preview" function is activated so you can see the changes that you're making to the skin tone immediately. There are no ideal settings for this filter. simply play around until you feel comfortable with having cleaned up any sking blemishes in your subjects face or body.
You will have lost some detail in important areas such as the eyes of your subject as well as any areas with hair. This is easily fixable by ensuring your second layer is active and clicking on "layer -- layer mask -- reveal all" (found at the top of the Photoshop window). You need to use this mask to expose the textures in the eyes and the hair. To do this simply choose black as your foreground color and paint with the "brush tool" over the objects eyes and hair. The original texture of the image will become visible.
However, you'll probably notice that your image has a slightly plasticized look to the skin. Although the skin blemishes are fixed, the image now looks unnatural. In order to fix this you need to expose some of the background skin texture. To do this simply ensure your second layer is active and then use the "opacity" level in the layers window to expose some of the original skin texture. Usually a setting above 50% is desirable because it allows you to maintain the effects of the cleaned up skin while at the same time making the photograph look natural.