As I've said the well placed city is first step of city specialization. The second step is to construct appropriate landscape improvements.
The Commercial City
Cottages. A lot of them. This is absolutely most important improvement to build. If particular city has even low but reliable Food surpluses, it's a good idea to create Cottages on every possible (and used) flat tile. Cottages are a very specific kind of improvement because it takes a lot of time to fully develop them into town (70 turns).
On hills you can put Windmills - they give some commerce and a good amount of Food.
Nevertheless it is good idea to leave some tiles only for Production improvements (Mines, Lumbermills, Workshops, Watermills). As I've said, even the most advanced city needs a few hammers - without them it will take ages to build all the useful buildings.
The Production City
Two types of improvements: farms and all Production enhancements. We start with mines and forest only so I advice avoiding chop rushing close to productive cities. In later game Workshop, Watermill and Lumbermill become available and can help us in generating more Production output.
Usefulness of Workshops is strongly connected with your technological advancement - at first this improvement is only useful when you have no other means of giving more Production to the city (no forest, no hills). After inventing Guilds and Chemistry Workshops become much more valuable.
The one of the most important Production improvements is Lumbermill (available with Replaceable Parts) witch gives +1 Production from terrain (forest) which already generates additional Production. Railroad is a very balanced (bonuses to health and usually some Food Production) improvement - this is the main reason to limit the chop rush.
The Railroads are very handy too. Lumbermills and Mines with Railroad has +1 to Productions which makes it's the most productive improvement in the game.
The Great People Factory
As I've said the intention of GPF city placement is to maximize of Food harvest. Every city's resident uses 2 Food, what means that to maintain specialists you need two residents working on grassland farm (generating 3 Food) for every specialist. That's why Food resources and flood plains in case of GPF are so important - they make it possible to have more specialists for less number of Food generating citizens.