Rules for the GM
These are rules to strive for. I'm not the best at following these rules, but I know that these are the things that make for a good GM. I point this out now, because this next one is something I'm always trying to improve at.

Rule #5: Describe! The players can't see the world unless you give them a description. Give details. The more you tell them, the better they can "see" the scene and the more the game comes alive. Try to give colors, textures, smells, etc. It's not neccesary to spend a lot of time setting the scene but by adding a few sentences you can really add a lot. Try to make the details fit the character. If a character is known for spotting clues like Sherlock Holmes, make sure they notice something that noone else would have noticed. If a character has an extraordinary sense of smell, describe to the player a faint smell in the scene. Something like "the faint lingering smell of an incense stick that had been burned in this room within the last couple of days" can really help that player get into character. Likewise, if a character has a hobby or intererest, try to give the player a detail that has to do with that interest, something another character may not care enough to notice. If they are in to plants describe the plants that are nearby or the lack of flora.

Remember to describe things from that characters point of view. If you use terms or words that the character would not be familiar with, the player may know what you're describing, but the character won't have any frame of reference. If a character from a low-tech setting sees a skyscraper try to think of what it would like to that character. That skyscraper, if seen at night, could look like a "huge dark tower that reaches up so high it dwarves any tree you ever seen. It's surface reflects the moonlight light as if it were made of a dark stone that's highly polished." As the character approaches he may realize that that dark tower is "made of the smoothest glass you ever seen and is somehow made of this glass."

If there is likely to be action in a scene, be sure to describe items nearby that could act as cover or as a makeshift weapon. This allows the player to be more descriptive in their actions. Instead of "diving for the nearest cover" the player can "run to the end of the dock and leap behind the shipping crates." Instead of "picking up the nearest thing I can use to defend myself", the character can "throw open the door to the broom closet and grab a mop."