I learned to GM by observing others, reading game material, and flying by the seat of my pants. And, yes, I continue to learn more each time I play and each new game I read.
Burning Wheel is currently schooling me big-time.
I had been watching some kids play AD&D at my summer camp. I was totally obsessed with knights at the time, so the game hooked me long before I even understood what was going on.
The first time I actually played, I DM'ed
Keep on the Borderlands with blue-book BD&D for myself and my buddy Hassan. I think it was 1980. We both had characters (elven f/m-u's, even though we didn't know you couldn't do that with Basic), and I was pretty "active", e.g., "The minotaur is in room 34. We should go there!" I had no clue how the rules worked.
Anyway, buying other RPGs (
Runequest,
Champions, and
Traveller were among the first) provided me with a lot of insight. You read mutiple "What is a Roleplaying game?" and GM advice chapters (from different authors) and the skills start to sink in. Subscribing to
Dragon was also critical.
But, most of all, in the pre-Net era, observing other GMs was the most useful for me, and has generally provided all of my RPG epiphanies. In junior high and high school, my friends Evan and Dave were my teachers; from them I learned a lot of the basics. In college, it was Mike C. and my roomate Fred. Mike C.'s
Call of Cthulhu sessions were like being taken out of the cave and into the light, GM-wise. Then, more recently, it's been the GMs in my current groups. My
Champions GM, Mike L., blows me away pretty mcuh every session; he doesn't even bring any written notes! There have also been some great GMs at ENWorld Gamedays (CMG Mark and msasso, I'm looking at you) that have knocked my socks off.
Online communities have also taught me a lot. Being able to pool the collective knowledge of thousands of gamers all over the world is just... I really can't remember how it is I survived before the 'Net.
Whether all this has produce, in me, a good GM, I dunno. I'd like to thinkk so, but you'd have to ask my players.