1 - NEVER put your Ego before your players needs.
Example - So, you spent an hour creating that particular NPC - and another two hours painting him. So what if the players finish him off before your expected, thats the nature of the game - don't go out of your way for revenge or for the NPC to avoid being killed. Grow a Sack and be fair!
2 - Know your Game!
Example - Nothing BUT nothing destroys confidence than a DM thats constantly looking up rules in the books. Screens go a long way to resolve this, but a scratch pad is a wonderful tool. Any rules you aren't sure of, write them on the pad - no need to check out the book then. Also, if you make a house ruling - write it on your Scratch Pad. That way you can be consistent because you never forget anything!
3 - Never let the Rules get in the way of the Story.
Example - If for some reason the rules themselves get in the way of story flow - ignore the rules. Make a quick note on your Scratch Pad (so you don't forget) and carry on. Refer back to your Scratch Pad between games and rationalize your decision, or make it part of your core way of running things - NEVER explain things during a game ("all will be revealed" is my favourite phrase) if you do waver you can come over as uncertain (see rule number 2).
Beyond the above, make things as fun as possible - for both your players and yourself - there is nothing worse than a bored DM! These are generic ideas that shouldn't interfere with your individual DM'ing style!