Hiya!
1. Be Painfully Neutral and Fair.
2. You are the Reality of the Campaign and everything in it.
3. Whatever happens...THAT'S the story.
With #1, it basically means don't "build for the PC's to succeed". Meaning if the Troll Swamps is being crossed by the level 2 PC's...and you roll an encounter with 6 trolls... SHOW NO MERCY! It's the Troll Swamps for a reason! If the Players want to believe in your world, their PC's and the fiction going on in it...you can't just "re-jigger it" to be a trio of stinky orcs. You set up the Troll Swamps, the Encounter Charts for said swamps, and gave the Players every opportunity to have their PC's do some research as to 'why' the swam is called the Troll Swamps. That was fair. Rolling the dice and letting the chips fall where they may...that's the neutral part. Too many DM's are focused on "fixing" unfair situations for Players/PC's that they might as well just be telling the Players what happens.
For #2. You are the CAMPAIGN...you are NOT the "storyteller" (see #3, below). You run the 'world'...and the world is NOT built for the PC's. You have an infinite supply of bad guys, dangerous situations, and everything else you can imagine. Don't focus on the PC's...focus on the "Reality of the Campaign" and play the role of the gods. Weather, seasons, wars, religions rising and falling, famers getting a bumper crop...or having a famine wipe them out, guardsmen with crushes on the local milk maid that comes into town every morn...that's your job. Your job isn't to be the "architects" of the PC's lives. Let them live in your Reality and react accordingly.
Lastly, #3. This one is pretty important nowadays, with so much emphasis on "The PC's are the Heroes of the Story". News flash...no. They are not. Not in the sense that it is thought of today. The PC's are the heroes of the story...THAT THEY ARE TELLING! Meaning DM's shouldn't be setting up "When the PC's do A, B happens. Make sure A happens. It MUST happen or there is no B and the story to C is ruined". Wrong interpretation of "PC's story". That's the "writen story" or the "DM's story". If the PC's do A, and B happens, but then take a 145 degree turn away from C...well, THAT'S THE STORY! Go with it. Enjoy it! Be surprised at the wonders that the Players are bringing to the table. So "C" doesn't happen as expected. Remember #2? You being Reality? Yeah, what would "realistically occur" now that the PC's have ditched the whole "C" part of the adventure? Cool! Go with that! Don't try and get them 'back on track'...because then you're telling YOUR story...not letting THEM tell YOU the story.
^_^
Paul L. Ming