Use a make up artist. Seriously, it will save you hours in Photoshop... This way you won't have to use many of the 'bad' skin smoothing methods on the web that incorporate some kind of blurring.
Learn your blend modes and how to leverage the adjustment layers you can reduce a lot of skin issues using these before you start to do destructive editing.
The key here is not so much remove a lot of flaws but make them less of a distraction unless of course you're going for a high fashion/beauty look. In all cases start studying human skin and what actually makes up skin. It's a lot more varied than people realise.
Here is the technique I always use for smoothing skin -
Duplicate the layer
Use high pass filter (3.5 for low res - around 40-45 for hi res)
Desaturate the layer
Invert it
Add Layer Mask
Invert Layer Mask
Set blending mode to Overlay
Use the brush tool at 70% opacity and bring back the areas you want to softten up.
When a makeup artist is not handy, one basic way to smooth skin is to create a base layer with the image, a second layer which is guassian blurred, and then a layer mask on the second layer. You erase the layer mask so that the second layer is 'transparent' on the first image. Then you fill the layer mask in on spots of the skin that you wish to smooth so that the second layer "shows through" on top of the first layer. In other words, the blurred portions of the second layer replace the rough portions of the first layer to smooth the skin. Of course that is a very basic technique. You would want to selectively control your blur type, level transparency and level blending to achieve an optimal effect.
It's kind of hard to explain in words, you should just try a Google. I'd say there are dozens of effective skin smoothing methods. Some are better than others.
Fix larger blemishes with the clone tool, healing brush, or spot healing brush. For overall smoothing after that, here is a technique that's easy and can be built into an action. If you want to buy a somewhat pricey plugin, Imagenomic Portraiture is first-rate.
Any blurring/masking technique makes skin look smooth, but completely unrealistic. The real way to do it is dodge and burn at with a 2-3 pixel brush at 3% opacity. A full retouch on an advertising image takes anywhere from 2-10 hours (or more!)
