Game Mastering: Preparation

Preparing for a game isn't fun. I'll be the first to admit it feels a lot like work. But anything that's worth doing is worth well, and that means taking the time to do it right the first time. Here's what you need to do to make sure you have everything ready for your game.

Read through the entire adventure. All of it. Whether it's reading a premade or going through your own notes, intimate knowledge of the adventure is going to be vital to a successful game. Mark pages that you know you'll be going back to, like stats for characters, maps of different areas, or special rules for an environment.
Read through anything else that's referenced in the adventure. If you have a gray tentacle goblin attacking your players, make sure you know the special rules for it's paralyzing tentacle caress, and mark the page you found it on.

Have a plan B, and know it as well as you know the primary adventure. As much as possible, anticipate what the players could do in a situation, and be ready for it. If there's a town two days travel from the adventure that's not involved in any way with the quest, be ready for your players to go there. Have something for them to do, even if its just a hook to get them going the direction you want them to go. If there's a minor character that the players interact with, be prepared to answer questions that aren't pertinent to the adventure. This is especially important with premades, where they don't always give you a lot to go on.

Have anything random on your end already rolled up. If there are random encounters, roll those ahead of time, read the entries, and mark the pages. Likewise, roll up any random treasure ahead of time. You don't want this to happen: "You've slain the dragon! In his horde you find....hang on a sec ::rustle rustle clatter::...I could have sworn the treasure chart was in that book. Hand me that Game Meister's Guide...No, the 2.7 edition one. ::mrEOOW!:: Sorry, fluffy." It's annoying to the players, it breaks up the flow, and it's hard on the cat.

The goal is to be able to run the game without interruption. At no point do you want to have to stop and look something up, because that's time the players are spending not playing the game. If you prepare for the game, you'll have a better session, your players will like you more, and you'll probably be asked to run more games. That last one may not be the incentive I meant it to be.