1. Every time should be the first time. That's the ideal mindset, at least. Each time a new game starts, try to make it the first time everybody has played it. This cultivates a feeling of freshness and exploration, and with time, players may feel they've explored the world once they've actually explored some of the world instead of instantly knowing it before the game starts.
2. Describe it like *you* don't know what it is. When you say the word "goblin", a host of information comes to mind. Stuff like hit points, weaknesses, common stomping grounds, and their favorite Led Zeppelin album comes along with it. But when you, the GM, describe a "feral, tusked person, with green skin, molting and shedding what little hair is still on his head, drool dripping from his lower lip and reeking of month-old waste..." Well, that could be anything. Even a goblin.
3. Be in the moment, every moment. Try to keep players interested in what's happening now. More importantly, try to keep them interested in what's happening *next*. Now that you have them exploring this new game in a new world (so to speak), now make it worth their time. They'll thank you later.