"You see in front of you the entire city, trees full of green leaves watch the city. Many people go from place to place, making the city seem full of life and unresting. Large red areas fill the remains of the surroundings, as the enemy siege towers grow bigger and bigger as they come closer to the city..."
So, today I wanted to talk about a thing that was overlooked by me for many a years, and I'm still seeing it in many GMs' description: Exciting all the senses. Every literary character, and PCs are included there, has 6 senses: Sight, hearing, smell, touch, taste and the sixth one. A mistake that is commonly used is to forget about the last four, and to go almost solely for sight with a little help from the hearing sense. The description from above is an extreme example of this: Only sight.
Tip 1: In every description, excite at least 2 different senses.
Tip 2: Don't go for the same 2 senses every time, make as much movement in the 2 you go for as you can.
The sixth sense, which deserves a paragraph of its own, is the feeling sense: "A dreadful feeling fills you..." This kind of things. It's a sense that should be used, but it needs a special consideration. It's always better to say that this place makes you want to puke than terrifies you. It's even better to say that "it seems too easy", or "too silent", etc... Go for the sense, but don't state for the players how their characters really fill, make the players do this "dirty job".
Another thing, that is often overlooked, is the description of how someone speaks: "She speaks with anger as she says this", or describing an atmosphere of tension. It's far better to give it through the voice. Talk angrily for the first example, or squeak for the second, and the tone and atmosphere will reach the character quicker and far more powerful.
Tip 3: You can use your voice to convey anything that you want it to convey, including contradictions to what you say, use it.
Lastly for today, is "how many details should I include in a description". Truth is, there's no definite answer. It's usually better, though, to make descriptions short and to the point. Don't be afraid to give just the surface and to expand the description according to the players' questions: "A smell of blood fills the room. -From where is it? -You go after the smell and you find, behind the table, the body of Sir Lancelot!"
Tip 4: Make descriptions short and to the point. 3 things in 20 seconds should be enough.
Tip 5: Don't be afraid to expand and/or to wing things on the fly according to the players' questions. It seems much less of an overdose...