Over the shoulder of my sister while she was learning Graphic Design in school.
Then lots of messing around on my own.
My skill curve really only shot up though when I started doing actual client work in a small ad agency. I think that it was a combo of harsh lessons in being practical when it comes to graphic design (losing the notion of this is "my" design and I'm making art!) and also the environment of being sandwiched between two brilliant other graphic artists who kept giving me suggestions and criticisms all day long.
But I think as a graphic artist, a bunch of general knowledge in all forms of art is a really good thing, because inspiration comes from all the weirdest places, and thinking that one should only look at or read one kind of thing is a good way to stunt one's artistic growth.
Oh, and Datura. She's not on Sitepoint much now, but I've learnt tonnes from her criticism.