Having DM'ed for 20-odd years now, I have found that the following guidelines generally make things easy for everyone, players and DMs alike. The number one goal of gameplay is to have fun. To reach that primary goal, things need to keep moving, and so speed and streamlining is goal number two. Complete adherence to the written rules is tertiary to the above two goals; i.e., if we can't have fun, keep things moving AND adhere to the rules, the rules are the
first thing to go.
1 - I will be honest and up front with my players about my rules knowledge - to wit, any time a player tries to do something and I know that I don't know the specific rule to handle it, I will immediately make that known.
2 - If I make it known that I don't know the specific rule to handle it, the players have exactly 30 seconds - no more - to give me a page reference (PHB/SRD only) that handles the situation.
3 - If no player comes up with a page reference in the given amount of time, I make a ruling "on the spot" that is specifically NOT precedent-setting past the current gaming session. I make note of the situation and the rule in question; after the session (maybe later in the week), I research the rule. At the beginning of the NEXT session, I advise the players of the correct ruling, and it is played that way from that time forward... but there is no "ret-conning" of results if I goofed.
4 - A player that has a disagreement with my interpretation of the rules is welcome to take me up on it after the session. If I do not say I don't know something and he sees me doing something wrong, he is NOT to interrupt the session; rather he is to talk to me afterward and educate me. In other words,
if I didn't ask for help with the rules, don't try to quote the rule to me during play. If I do have a bad understanding of a rule, again, I announce the change in ruling prior to the next session - but again, no "ret-conning."
I make it clear to my players that they are welcome to discuss my rulings with me after the gaming session (though there will be no ret-conning) but are NOT welcome to bog the session down with rules discussion. When we get together, the point is to play the game, not to argue about the rules.
To extend the sports analogy, it is the job of the referee to make "on the field" (i.e., mid-game-session) rulings. If the referee's understanding of the rules is lacking, the time to correct that is NOT during gameplay, as this slows the game and ruins the fun of others as well, as they wait for the argument to subside; the time to correct the lack of understanding is "off the field" (i.e., not during game sessions) - this does not disrupt the game.
A player letting me know when I've goofed is appreciated, provided he does so in a manner that does not disrupt play. In fact, I've even given credit where it is due and told my players, "so-and-so pointed out Rule X to me this week on page Y of the PHB, and I've been doing it wrong up until now. Going forward, we will do it this way..."
A player who wishes to disrupt the game - for ANY reason (including mandating strict adherence to the rules) is not welcome at my table. A player who is more knowledgable than me and is willing and able to help me increase my understanding of the rules - provided he does not disrupt my game - is MORE than welcome, and is in fact a valuable resource.
Finally, I always make it clear to my players that I may NOT be using the "Standard MM monsters" or what have you; I may have made my own modifications, so the Monster Manual is NOT necessarily the definitive Bible (i.e., "we did 60 points of damage and the MM entry shows a maximum of 55 hp - it's gotta be dead!" won't work with me). Note that in the above, I have only allowed players to use the PHB for reference. In other words, you're more than welcome to tell me how YOUR character should work, but since you don't have MY character sheets in front of you (monsters, NPCs), don't try to tell me how MY characters should work - you may well be very wrong.
--The Sigil