While all of the other writeups here are good, solid advice as to how to run a roleplaying game from a DM standpoint, one must know that as the host
of the game itself, there are guidelines that should probably be followed for maximum enjoyment
One of the most important factors in hosting a game, whether it be D&D or Paranoia, is the space in which the game takes place. This can be anywhere from the living room to a kitchen table to the basement. While the goal here is to make sure that everybody fits comfortably (or as uncomfortably as they're willing to live with), where you play your game sets the tone. Slaying dragons in that dungeon loses its' aura of magicalness when you're sitting in a brightly-lit room, trying to keep from staring at the grotesquely old family photos on the shelf.
While good lighting makes for good atmosphere, remember that it needs to at least be bright enough so that the players can read their character sheets without use of flashlights.
Again, the comfort of the players is important. Overstuffed chairs, couches, sofas, and so on are good for gaming. If at all possible, the DM should have a more comfortable chair, just because he's the DM. Remember to move all of the chairs and whatnot to the aforementioned Location before the players arrive, the last thing you want is to have a game interrupted by new arrivals having to find a place to sit.
Food and Drink
This is possibly the most important matter to be taken care of. You can play a good game of GURPS in the dining room, but if your players don't have anything to munch on, there will be a riot, possibly ending with the host's liver being served with some fava beans and a nice chianti. If you can't determine everybody's tastes, cover all the bases by having as many varieties of soda as possible, preferably a brand with a higher caffeine content such as Mountain Dew, Dr. Pepper, or Jolt. As for food, while Rancid Pickle touched on the wholesome goodness of twinkies, a bowl of chips or pretzels can easily be passed around, and won't damage your figure or complexion as much as two or three twinkies will. Many RPGers already have serious acne problems, and twinkies definately won't help.
Music is a good thing. Perhaps the LOTR soundtrack or some chamber music from the Middle Ages, or something more techno-y and futuristic depending on the game. Always have more than enough extra character sheets, you never know when a player will bring a friend without advanced notice. The same goes for pencils. Not pens, pencils, and not just pencils, but pencils with erasers. Delaying a game by tearing apart the house in search of pencils or a pencil sharpener is always an ominous sign of things to come, plus you have to clean up the mess you made while looking.
So, now that you know how to run the ideal RPG, what's stopping you from calling up a few friends for a voyage into the latest dungeon of your creation? After all, if you're reading this, you probably don't have much else better to do...