Lightroom
http://digital-photography-school.com/how-to-professionally-retouch-portraits-in-lightroom/
http://digitalphotobuzz.com/using-lightroom-brushes-to-soften-skin
http://photographyblogger.net/hidden-gems-in-adobe-lightroom-skin-smoothing/
Photoshop
http://www.photoshopessentials.com/photo-editing/smooth-skin/
http://www.creativebloq.com/tutorial/high-pass-skin-smoothing-photoshop-812591
http://www.peachpit.com/articles/article.aspx?p=2166716&seqNum=6
http://digitalphotobuzz.com/softening-skin-in-photoshop