I learned by just messing around with all the tools, seeing what works best for what, and as Vicky said there's some effects that can be achieved simply by applying filters and the like. The most dangerous thing about Photoshop though, is deluding people. Apart from the misperception that having a higher Photoshop version makes you better at graphics (I'm most comfortable with version 4), a lot of people also fall into the trap of using cheesy filters like the lighting effects and lens flares :fero:
I think one of the most overlooked set of tools in photoshop are the image adjustments, if you learn to control those, it really helps no matter if you're editing photos or doing digital art