1) Communicate with your players before the game, and learn what style of game they're looking for. It could be a cosmic horror story, a political succession crises, a gritty story of a village banding together to protect against an invading army of [Random Fantasy Enemy], but it can help to know what style of game your players are interested in.

2) Don't let the session get bogged down. There are two primary ways to mitigate this issue. First, make sure to be familiar with any rules you expect to occur in the session. Know the spells/maneuvers that are likely to be used, and if you know a specific section of the rules will be used (such as a Underwater Combat or a similar part of the game) make sure you've brushed up on them. It can help sometimes to have a few notes if there's too much information for you to memorize. Second, try and plan out what likely outcomes are, and how those outcomes will affect the game as a whole. Some people are fantastic at creating on-the-spot events, but knowing what's likely to occur can take a load off your mind.