There is the Layer Keyword Reference chart as your best start. The skins are .psds and you can always look at how others implemented design aspects like buttons, panels, sliders, animations, etc.
Googling photoshop tutorials is probably the most useful tool in my designing kit because it offers more than one approach to most techniques.
So I'll get an idea for a spotlight for example, and then look at 4 or 5 and see which techniques I want to use.Sketches are useful too.The thing to keep in mind is that there are often multiple approaches to achieve an effect or design.
Also, there are tons of free .psds and GUIs available, if having a basic idea and a few layers to start with helps. Take a look at Lances website or guiStuff (it's got a new name I think.Also, look in skin resources here..)
You can even download complete GUIs and .psds and that is a pretty simple way to play around and learn, getting fast results you can work with in like 5 minutes. Then you can learn trickier stuff at your leisure.
Lastly, there will always be new tutorials, but in the meantime, any question I ever had or problem with a skin, it was always answered in this forum.