Moldvay - "That's not in the rules!"

"The players will often surprise the DM by doing the unexpected. Don't panic. When this happens, the DM should just make sure that everything is done in the order given by the outline or sequence of events being used. Minor details may be made up as needed to keep the game moving. All DMs learn how to handle both new ideas and unusual actions quickly and with imagination."

My favorite part of this is "minor details may be made up as needed to keep the game moving", or, more specifically, "keep the game moving". Few things can derail the sense of immersion and involvement in a session than stopping to flip through a rule book to look something up.

This is probably one of the hardest aspects of DMing to "teach", it must rather be learned from experience. When faced with a dilemma, it is the DMs responsibility to react, quickly, with a ruling, rather than stop the flow of the game to look up, or worse - debate!, a rule. Its important to remember that your fairness, impartiality and sense of balance play a huge part here - if your players trust you and trust that your rulings are fair, even if they don't necessarily always hew to the letter of the rules, you're going to be able to implement this a lot more effectively.

Lets say, for instance, the party is battling trolls. One of the players, in the midst of combat, wishes to light a torch and start burning troll bits to stop a defeated troll from regenerating back up into the fray. In a "DM vs. Players" scenario, the DM would most likely be inclined to favor getting "his" troll back into action to whup up on the PCs some more, and tell the player its going to take a couple of rounds to light the torch, a couple more rounds to effectively burn troll bits, and all the while suffering the undefended attacks by the remaining trolls.

Unfortunately, what this DM is actually doing is punishing creative play. "Get back to unimaginatively rolling 'to hit' and damage", he might as well say, "or you're going to get pounded on."

This results in one or both of two things - the player demanding rules to back up the difficulty of burning some troll bits, and/or not trusting further rulings from the DM.

A little leniency in situations like this can go a long way towards not just getting the players to trust your rulings, but not have to worry about stopping the action to back up every single ruling, and it encourages more interesting and creative player actions (ie. cutting back on the "roll play"). Make up your "minor detail", such as "a torch can be lit this round, and you can either move somewhere with it or burn something next to you", and make sure this fits into the sequence of events, and get on with the game.