Hill Cantons ignited an interesting discussion on Building a better GM.
First I read books and watch movies. Both for enjoyment as well as keeping an eye on how they do things. Twenty years ago, I gamed with a guy named Wes and we went to a couple of movies. He was a movie buff in addition to being a gamer and he had a gift for explaining what was good and bad in a film. It was an eye-opener for me and I never looked at movies in the same way again. Afterward I learned a lot of little bits that I incorporated into my game. Mostly dialog and how two actors played off of each other. I also read lots of books and found them a great source of inspiration.
I do this to build up a bag of stuff in my head to pull out while running a game. A bigger bag of stuff means you have more options for when the player talks to the tavern keeper, arrives at royal court, or just stumbles across a peasant's hovel in the middle of the forest.
The key thing is to take some quiet time and organize things in your head. Think about a peasant, an orc, a kobold, etc and then ask yourself a series of what if. There is no right answer and you don't need to be Star Trek's Data and have perfect recall. Do it enough times it will come to you when you need it. This technique shares some ideas with that of the Memory Palace. The best part is that you likely have much of what you need already due to being exposed to fantasy and other genres over the years.
Another technique is to challenge yourself. Conventions are perfect for this. Run some situation, genre, or game that you never done before. The idea isn't to change your style of play but to broaden your experience. By experiencing different methods of roleplaying you will understand the ones you like better. With Google Plus gaming and the virtual tabletop games there are more opportunities today than ever before to try different games at a convenient time.
For me it was playing and running a LARP game for over a decade. Writing and running adventures for LARPS is stifling. It is because the physical logistics precludes the flexibility of locations, creatures, and items, that you have for tabletop. Despite the limitations I learned to write fun adventures for LARPS and it helped my tabletop game immensely.
Another thing I did was run a dungeon adventure cold. All I had was the stats for the big bad guy, the map, and a general idea that it was a coven of warlocks. I just made it up on the spot as the players went through the dungeon. Knowing that it was a coven keep things from getting incoherent.
Also surprisingly running Tegal Manor. The sparseness of the description, (a room name and maybe a sentence or two) has really forced me to reach deep into my bag of stuff to keep the game going. I always done it with my sandbox campaigns but since Tegal is a funhouse dungeon it forced me to go outside of the things I done before.
On a different note, I been refereeing large groups of players at my Gold Star Anime games. One session I had ten players. This is something I hadn't done since the early 80s so I had to relearn a few things. Once thing I did was figure out how to keep individual initiative. Basically everybody roll for initiative. I roll once for each group of monsters.
In my games the DEX bonus gets added to the die. Fighter get to add their to hit bonus from their class. For monsters I add 1/2 hit dice. So I start at 10 and count my way down calling for anybody who has initiative at that number. I found this works just as fast as doing single die initiative and going round robin around the table.
Split groups don't bother me, I just go round-robin between each group giving them 5 minutes at a time.
There is my How to Build a Fantasy Sandbox for preparing a campaign.
I always have players speak in first person when they speak as their characters. I find it helps immersion.
I do the funny voices, I also practice so I am pretty comfortable doing them. I understand it is not everybody cup of tea.
If you use miniatures, the best thing you do is take all the most commonly used items; beds, tables, orcs, guards, chests, doors, etc and put them in an easily accessible box. I use foam lined bead trays bought from Wal-mart.
Plot in my campaign is a time-line of events that would occur if the players don't do anything. It gets changed (or not changed) after every session to reflect what the PCs did (or not do). I have a roster of NPCs. The creativity comes from deciding how the NPCs react to the PCs.
I try to sit down with each player and do a one on one session to develop their character background. Then incorporates elements of that into the campaign's plot. I make sure there are as many potential benefits as there is potential complications from doing this.
I have way more than the three Hill Canton asked for, I hope you find them useful.