If you flip to the back of Moldvay's Basic book, p.B60, you get a nice 3/4 page detailing "Dungeon Mastering as a Fine Art".
It covers the following:
"That's not in the Rules!"
"There's always a chance."
"The DM is the boss."
"Everyone is here to have fun."
"Everything is balanced."
"Your character doesn't know that."
Seeing as Moldvay managed to produce, in my opinion at least, the most concise and user-friendly version of the D&D ruleset, I feel it's worthwhile taking a look at his opinions on the more ephemeral and arbitrary elements of running a good session. So I'll be writing a bit about the above elements over the next few posts.
Introducing this section, Tom says:
"It is important that the DM be fair, judging everything without favoring one side or the other (BtBG - something I like, and talk about a bit more here). The DM is there to see that the adventure is interesting and that everyone enjoys the game. D&D is not a contest between the DM and the players! The DM should do his best to act impartially when taking the part of the monsters or handling disputes between characters."
The bolded portion above (and this is bolded in the actual text, not by me), is deservedly so. Again, as Tom does so well in every aspect of his little corner of the D&D library, he sums this up very concisely. Its a little like dropping two scorpions into a jar and letting them duke it out, only one scorpion is the players, the other is the adventure you've designed, and the jar is your campaign setting. Fudging/cheating in either party's favor defeats the purpose. The reward for the DM is not "beating" (or coaching, or assisting) the players, but rather the pleasure of watching all the variables and challenges the players bring to the table unfold as they react to your creations. As Tom mentions, there are many players, and only one DM!
I behooves those of us who are DMs to take this advice to heart.