If you can't describe what you're writing about, then perhaps it's not worth writing, in the first place. Had the same problem plenty of times. Write what you know early and often, it'll become second nature for you in time. For now, flesh out some detailed enviorments you are sure about and consider what makes them interesting. Apply this to whatever story is in question.

 

I have to agree with Fastkilr on this one.  "Write what you know." 

The descriptions you did offer ("the dew shining in the lazy sunlight, the crunch of snow under your boots as winter slaps you first thing in the morning") are not location-specific.  They could describe any city, weather permitting.  It's best to stick with that type of description than to cobble together a description of a city from the Internet! 

Surely you've done some traveling... Describe only those cities with which you are familiar.  And if you've never visited any of the world's great metropolises... it's time to pack a suitcase and do some field research!  Readers can always tell whether or not the writer has ever left his parents' basement ;) -- or not gone farther afield than her favourite out-of-state outlet mall  :P

Bon voyage,

RW

P.S.  I won't bother to directly answer the other part of the question "How can you 'punch up' descriptions of cities that are fictional?"  --  That's like asking us to tell you how to write, period.  All I can say is that, in a work of fiction, a city is a character as important as any other.  Check out "Cities on the Edge of Time" by Vivian Sobchack.  Her piece deals with sci-fi films, mainly, but we all know that good cinema has to have good writing. ;-)

When writing about Iceland, I used a variety of techniques. I wrote mostly about landscape, but the same things would work for cities.

First, I read a lot about the place: history, culture, literature (classical and modern), anything to give me a feel for the area. Next, I looked at a lot of pictures. Descriptions based on pictures are filtered through what I learned while reading, so the culture influences the descriptions.

Then, I did something that some people might not think about. I contaced a professional photographer who has worked extensively there, but is not native. Not being native helps a person see things that natives are used to and don't necessarily see. I asked about the light at different times and seasons, textures of stone, anything I could think of (and much more than I needed) that a photographer might look at differently than a tourist or native. Interviewing a painter would probably work just as well, or maybe even better.

I also had a native check my descriptions to make sure they felt authentic. This helped me find errors in my descriptions of weather and some other problems, and helped validate that other things were correct. 

The important thing is getting a sense of the place. You don't have to describe anything exactly, but you have to make it feel real. As with all research, you'll probably only use a little of what you learn, but that little will be informed by the rest of the stuff you don't use, so you can make a little detail do a lot of work.

For real cities, you research.  Write to or call the local Chamber of Commerce and get all their information for the area.  Request national real estate brochures for the area.  Review them, and look up schools, businesses; you'll get the flavor.  Look for movies set in that city.  Look up websites for anything in that city to get snapshots of various businesses for style; look up travel blogs for mention of the city and impressions.  Get the local newspaper and review the police blogger, the births and deaths, the local news items.  Once you have an image in your mind's eye about what it looks like and what it IS, really, like, it's easy to add descriptives to your narratives.  For imaginary cities, do the same with a city of the same size in the (roughly) same area, e.g., if your story is set in the South near Charleston, go with Charleston's issues and local flavor.  If it's fiction, better map out what you do put in your story so you don't make mistakes with what is where, and so on.  Best wishes, Barbara

With my book length manuscript Twinheart, I was writing about a town I had visited only once as a child the town of Lancaster and Palmdale.

My mom remembered it she lived near there as age fourteen in Littlerock, and I asked her a lot of question like was it really cold in winter, because my story started in the fall of the year. I also did a lot of research on the internet, and down loaded pictures taken by amature and professionals a like, and read what people wrote about there home town on the housing blogs. I also bought books on the deserts of Southern California and a city map of Lancaster and Palmdale so I could describe the streets. I also checked the weather maps, and such so I could describe the weather too.

I can't really say anything about writing about real cities one has never visitied that hasn't been said.

Ficticious cities, however, are up to the writer as to how to design them, and, therefore, require more work.  You have to make sure when you put a coffeeshop on the corner of 5th and Pine in Chapter 1, that same coffeeshop can't be at 6th and Elm.

What some writers, including Orson Scott Card, have done is draw maps of the city and refer to the map as they write their story.  And, in drawing a map, you might learn more about the city and your story. What kind of controversy did the Interstate running by your city cause locally or statewide?  Did you close off an entrance to a fantasy story's city?  Why? 

  

There are two common reasons for setting a story in a city where you've never been: one is because you think where you are isn't very interesting or exotic, the other is that the exotic location is vital to the story. If your problem is the first, then you are wrong. Any place you know well is interesting. The quirks of your town, it's politics (conservative, everybody knows everybody's business), economy (agricultural? fishing town?), climate (hard winters and bad storms) influence how people behave and determine what is normal, and better, what is not. Remember that one of this country's most successful writers sets many of his stories in Castle Rock, Maine. The other reason for an exotic location is valid. Dan Brown's Da Vinci Code had to be set in Paris, and London. The locations were actually key to the story. If that's the case with your story, you have received good advice from the others. Also remember you are a writer now. You can fly to these places to do research and write it off as a business expense.