That guy did make his tutorial kind of complex, but it's still a good method. You can skip a lot of it (that whole bit about the channels), go right to the curve levels, by selecting layer/new adjustment layer/curves. It'll ask you to confirm, hit okay, it brings you up to that curves graph the guy talked about, like this:
Drag the little line on it down, that puts like...an overlay of 'burn' on the picture, makes it darker. Hit okay again and then ctrl-I to invert the layer. That kind of hides the effect until you make it visible by painting it (make sure you're on the right layer) with a white brush...the higher the opacity of your brush, the more effect you can get with it, so it's nice to use a soft brush and have the opacity set kind of low so you can build up the darkness in some places, and just go over it lightly in others. So what you'll be doing is enhancing the darker areas, making a nice contrast for the lighter spots you'll do next.
So again, go to layer/new adjustment layer/curves. This time pull
up on that line in the graph, make the whole thing lighter, hit okay. You'll need to hit ctrl-I for this layer as well, then paint it in the same way (soft white brush), but concentrate on where the light highlights the skin because this layer enhances the light instead of the darks. The best thing for the 'oiled' look is to try to get the highlights and the lowlights close together, it gives a lot more of a reflective feel to the skin. That's the whole point of what he was doing, was enhancing the lights and the darks so there was more of a contrast, to give it that feel.
And of course, you can smudge or erase any mistakes on the appropriate layer, and it doesn't mess up the background and you can go over it again if needed. Another easy way to deepen shadows or highlights you've made this way is to do all the above, then duplicate one of the curve layers to see if that would get you closer to what you want.
This method really does work pretty well. I use it a lot and it's easy once you get used to it. You can also play with plastic wrap filter to make water streaming, that works well with skin as long as you apply the filter to small areas only (better to copy/paste a selection, so you can play with it on a different layer).