Personally when writing a story, I usually will get up walk away from my computer, and take a walk outside. The walk usually gathers my thoughts and helps me compile ideas better so that I have a much easier idea in mind for a game, but then again I can get distracted easily. Second, Bookmark important or commonly looked upon rules in your book or pdf,...can you bookmark a pdf...anyway,: combat, equipment, spells, etc, I'm sure any DM will tell that even though they've read a core Rulebook cover to cover most of them don't have photographic memory so it doesn't hurt to make it easier to look up a rule, either obscure or common, when you or a player are confused about something. Finally, have fun...I know that sounds cliche, but come on it's obvious, especially for the DM, I mean that guy/girl runs the show and if they aren't having fun how the heck are players going to have fun? I mean most players, and I say most because those that aren't considerate enough should probably play PbP instead of tabletop, are not going to run you into the ground if you make a couple mistakes as long as they have fun. Honestly I probably have more advice but I'd think it'd be better to see what others have to say.
While on the subject what RPG are you using?