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I'm far from an expert, especially when it pertains to retouching faces, but for skin softening, the best way to do it is with brushes. The most often recommended method to soften the skin in LR is to bring up the brush, set it to soften skin, paint over the areas you want to soften, and adjust the sliders to taste. Also pay attention to sharpening and masking sliders. If you're really into touching up faces, I would recommend looking into photoshop.
As far as plugins, I myself use Nik's dynamic skin smoothener in their Color Effex program. I find it works great as a one-touch solution (with default settings dialed down), but I can generally do a better job if I spend time working with brushes. However, I'm terrible and slow with brushes so I'm not always willing to spend the time. But give those brushes a try.