When creating the adventure there are a few important things to take into consideration. First of all make sure the adventure is something the players would want to take part in. The game is supposed to be fun for everyone. It should not be the gamemaster versus the players.
You should always encourage group play, it adds to the fun of all involved. While solo adventures can be fun, it is best to try and avoid too many internal struggles between characters. After all, the characters in a group are supposedly on the same side. Too much dissent can ruin a good game.
You should be familiar with the rules of the game before you try to run it. Nothing can slow down a well paced game than a game master flipping through the book for a rule. This doesn't mean you should memorize every detail of the game, but you should know as much as possible.
Be creative when running the game. Describe things to the players and interact with them. Don't make everyone sound the same, make every person the characters run into unique.
Even though this is a fantasy game, you should try to keep a little realism in the game. Don't let characters, or your own creatures, do things that just wouldn't be possible. You can let them try, but remember that realism makes the game more believable and enjoyable.
While you shouldn't try to kill off the characters, you should challenge them. If there is no risk, the game will lose its excitement. Death of a character is a harsh thing, but it is a harsh world.
Be flexible with the dice. You are in control. If a character is going to die and you don't want them to, feel free to fudge the die roll. Instead of having them die, make them lapse into unconsciousness or get captured by the enemy. This can also apply to your non player characters as well. If you have a truly perfect nemesis for the characters, don't let him die too easily. Think of all the movie sequels based on the ever popular reappearing villain.
Lastly, just because the characters defeat the main bad guy or achieve the adventures primary objective, doesn't mean smooth sailing for the way home. Killing something like a dragon is big news and will spread rapidly across the realms. Spectators will come out to gawk at the characters, thieves will try to get their fair share of the loot, and allies of the dragon may come looking for revenge. Keep the characters on their toes, surprise them from time to time.