Things You'll Need
Instructions
Prepare in advance. Before your game, make sure you know the important rules for any creatures or items you're going to have in your game. If the player characters have abilities you're not certain about, make notes on their character sheets to remind you.
Let your players know that you're running your game without a rulebook and ask them to bear the fact in mind if a difficult rules question comes up. You'll find that player groups will be very understanding.
Improvise. If a rules question you can't answer comes up, simply make a decision and move on. Make a note of the question so you can look up the answer later, but don't let the game slow down because you can't recall a rule. Keeping the action moving is more important.
Roll dice when in doubt. If you find yourself completely uncertain about a rule, roll a die to decide between the possible outcomes. D&D players are used to succeeding or failing based on the role of a die, and the dice are, of course, completely impartial.