A photographer I know recently shared some insight with me on retouching, this is what he said.
"There are lots of aspects to retouching (color adjustment, contrast control, removing lines and zits, softening the skin, reshaping the hair, reshaping the jaw, etc) and the best way to learn is to have someone show you. Less efficient is to watch one of the video podcasts. The information is great but you might have to sift through a lot of unnecessary stuff. Also, you need either Photoshop or some other program like it, like Photoshop Elements which is much cheaper.
Bad retouching is like bad makeup- it can ruin a great shot so I'd actually recommend hiring a retoucher who'll let you watch how they retouch. Then you can practice and compare your work to theirs. The actual techniques are very easy but getting good- and subtle- takes a lot of work.