I started in the fall of '02, lo, these three years ago...
When I started, I was terrible at it, and didn't know what the hell I was doing...and then my group of 4 (including me) joined a larger group with a semi-experienced (but still monty-haul prone) DM and abandoned our first campaign.
That second game folded in fall '03 when our party had killed off most of the major villains in FR, and I decided to try a Middle Earth campaign. It went pretty well, lasting from oct 03 to feb 04, when I killed it by making it too easy.
After that, one of the players DMed a short 3.5 campaign, in which I was a barbarian who killed things way too easily with a greatsword...ah, memories...
Then, I decided to revive my old homebrew setting from my first campaign. The campaign lasted from spring '04 until last march, when we finally got bored of it, and I realized that I was railroading the PCs a fair bit.
After that campaign folded, I did a few short games which never got off of the ground, including a dark-ages themed game and a near-future distopian setting.
After those, an old friend of mine and I got together and co-GMed a Star Wars d20 campaign over the summer focusing on Jedi in the period before the rise of the empire. I learned a lot about pacing and being tough on PCs without killing them.
Then, in September, I started a new incarnation of my longest running game using True20, which has been the most fun I've ever had GMing, and the most fun my players have had playing.
I've honed my skills considerably in three years, and I've gotten to the point where I know my strengths (plot, creepy horror elements, small town dynamics, and NPC interaction), how to play on them (my last adventure was about some evil cultists within an abbey who were robbing travellers by using dark magic), and my weaknesses (wilderness adventures, descriptions, and big battles) and how to counterbalance them (well structured notes).