What I try to do (success rate sometimes variable):
Be neutral. This includes no fudging, no on-the-fly changes to encounters if things get too easy or hard, etc. And most importantly, no favouritisim toward any given player(s) or character(s).
Be committed. This means showing up for the game on time (if not early) with my prep in at least some sort of useable condition. It also means running the campaign for as long - as in, as many years - as anyone wants to play in it.
Be consistent. This means sticking to in-campaign precedents and rulings, having clear in-fiction reasons for major mid-campaign setting or rules changes, and so forth. It also means being consistent with setting - if that wall had a door in it last time you were here, that door is either still there now or there'll be a damn good reason why it isn't.
Be malleable. This means that while I'll usually have a macro-level storyboard in mind, the players are always free to (intentionally or otherwise) chuck it out the window and go a different way; and I have to be ready willing and able to go into react mode on less than a moment's notice. Another way to put this would be "Be able and willing to wing it".
Make the game my own. This means if a rule or sub-system makes no sense to me, or is illogical, or potentially produces ridiculous or broken results in play, chuck it and if necessary replace it with something better.
Use every tool in the box, but infrequently or rarely and only when it makes sense. This means yes, I'll occasionally do a bit of hard railroading (e.g. "Yes you've just all been teleported halfway across the continent - you're someone else's divine intervention!"), or now and then coincidence might get stretched a bit beyond the breaking point (usually to get someone's PC into a party), and so forth.
Treat both the setting and the campaign as bigger than the characters. Characters and stories (and players) will come and go, but the setting endures and the campaign goes on. (exception: if the PCs in fact destroy the setting or game world - which I've seen done - that's it, game over)
The characters are native to and representative of the setting. This means the PCs aren't special snowflakes just because they are PCs, their specialness is earned through their actions. It also means the PCs are representative of their species' population, with corresponding benefits and drawbacks.