Here are a couple examples of futuristic city concept art pieces.
These represent three different television projects. The top being most idyllic and stylized. It is always a highlight being able to design skylines and cityscape concepts. These cities reflect a range from idyllic to oppressive and collapsing. The goal in these designs is to catch a harmony of vibrant forms and structures while keeping a level of density and credibility that captures the desired emotion of the population residing in the city itself.
While thinking about your city, ask yourself a few key questions during the process.
Usually I incorporate or decide on a hero structure or theme, a horizon, a camera angle, rough topology and design the sprawl and architecture from there. You can use very rough thumbnails for this step. The thumbnails below are a bit more involved than you should use for your very first thoughts. I suggest a wall of tiny thumbnail sketches using pen on paper, and snap a picture of it and drag it into Photoshop for very rough color and value studies. Don't overlook the value of these first roughs. Even though they don't take very long, every decision here will propagate down the line and will have a large impact on the rest of the image. Spend quality time, thinking about your composition, lighting and structures.
A tip using Thirds:
Like many illustrators and concept artist, I like to work strongly with 'thirds' in my image compositions. Above are a few of my thumbnails sketches divided into thirds. The trick is to avoid becoming uncaring or overly stringent. When artists always cut their subjects squarely down the middle in the third it can become heavy handed and overt. Try to mix hard edges, flipping gradations, as well as light and dark shifts on the third too.
The first question, and likely the most important is, what kind of inhabitants will it have? Your cities population will drive the vast majority of the descision making. What is the economy spread of the population? Are they organized or chaotic? Are they austere or gaudy? How long have they lived in the spot where the city resides? Are they technologically advanced? Did they build the city, find the city, or take over the city by force? The motives and history of the inhabitants of the city will drive it's look, and will inform you with precious, convincing and calculated detail.
Also note cities have behaviors that will help you create a convincing one. Generally people understand the center of the city is built up more than outlying areas. Value of the center of the city rises, and the only way to build is up. That said, you can go very deep while thinking about your design, in 1916 was the birth of zoning codes in New York. These codes give you a tool to think about while creating and organizing your own city design. Codes regarding height, types of industry and control could be subliminally modified to create unique and interesting looks. Also worth noting, racism, poverty, crime, wealth, industry and the concept of 'not in my backyard' are all moments in a cities lifespan that can have drastic effects on how a city looks and feels. If you find yourself stumped for a moment in your city, think back on the codes, and how the city got this way. This will help to inform direction.
Do yourself a favor and concentrate on getting great thumbnails first when going about rendering your city. Nothing can replace the value of a great thumbnail to start an image. There is no amount of rendering, lighting or detail that can turn a bad thumbnail into a great design. Once you have your killer thumbnail then decide on the best way to go about producing a final image. Typically, I have created rough 3-D layouts and painted over the top of them, however over time, people respond more to the images that are purely Photoshop. My suggestion is to use 3-d as a guide, but not as the solution.
The same goes for using textures and photo ref. I personally feel that you need to spend a good bit of time removing 'photo qualities' of your rendering if you use them. If you decide to use a lot of photo ref, make sure your surrounding rendering holds up to the granularity of the reference and you don't feel a split between your brush work and your photo layers.
Designing and rendering a futuristic city can be a daunting task at first, but with some practice planning and patience the process comes along quickly and efficiently.
All the best, good luck!
Worth a note, none of these images may be licensed or used for personal projects, these particular pieces get a lot of requests in that regard.