One of the most complex areas about retouching beauty shots is definitely fixing hair. If you see any editorial or advertisement that has perfect hair, you can be rest assured that there was some retouching involved. Although this isn't the only way to retouch hair, the guys at FX-Ray put together a good video showcasing some tips and tricks. There are definitely many methods of doing it, but this should come in handy for the common trouble areas.
More so than the techniques, what I actually enjoyed was the key point about getting it as right as possible before bringing it into post production. You can't circumvent the need for a good hairstylist to set a foundation. You can debate the results or the techniques, but the principle of good prep is universal with hair work. The key to a good final product is a strong image to start with.
You can also download the project files to follow along by going to their site.
Also as a quick heads up, I will be teaching a few great retouching classes at the Fstoppers Workshop in the Bahamas this May. Check out the details here (http://fstoppersworkshops.com/pratik-naik/. Come out and spend some time with us, you need that break! Check out the promo video on what you can expect at this year's workshop!