Following a thread I found at a norwegian forum, I've defined some of my own approaches to GMing. Here they are:
KISS - Keep It Sweet and Simple
Sometimes complex plots have their place. Just make sure you don't use them when they're patently not necessary. Likewise, rules are necessary to make a game work, but using too many rules will complicate things unnecessary.
Don't roll dice if a fumble is unacceptable
Live by the dice, die by the dice. If you're not prepared to face the consequences, make the difference not between success and failure but rather between qualities of success.
The PCs are the main focus of the campaign
Don't introduce hordes upon hordes of NPCs just to do so. Have them serve a purpose, and don't let them upstage the PCs. Make sure that you don't bottleneck or railroad your players unnecessarily.
Roll with the punches
From time to time your players will throw wrenches into the machinery of your plot. When that happens, let them happen. Try your best not to block. Remember that the players have a part in the campaign, same as you do.
Loners are not teamplayers
Yes; the concept is cool, but it rarely works well in collusion with a team. Likewise, a campaign is best suited in having similar characters.
First off, feedback means you get to know what you're doing well, and what you're not doing well. Second, it's an opportunity to change the focus of a campaign towards something that motivates your players more, differently or both. Make sure you use the feedback so that your players know that they have a say.
Take care of character sheets
I trust my players not to play around munchkining their character sheets at home. I don't necessarily trust them to bring them to a session.
Don't bullshit your players; be honest about your strengths and weaknesses. Use their feedback to improve.
Plan for flexibility
You should have a general idea of where your plot is going, but allow it to go elsewhere. Remember that the experience that each player has should always take precedence to the storyline and the plot. In the end, it's all about leaving your players with a good feeling, and preferably wanting more.
Be rested and prepared
Although routine and experience will compensate for a lot, GM'ing is still an activity that playes heavily on all your mental faculties. If you're sick, hung over or otherwise not on the top of your form, it is always preferable to postpone a session rather than run a bad one.