Wem said:
Can anyone offer tips on how best to specialize cities and what strategies to use, to that end?
So far I usually have every one of my cities emphasizing production, I usually have the avoid growth turned on at about 6 population for a while (anyone know at what point you can turn off avoid growth?) and eventually hit the emphasize great person.
Turn off Avoid Growth when the city has more

than

in it. Occasionally, you might want to leave it on anyways, but that's relatively rare.
The vast majority of your cities won't produce jack worth of Great People. That dinky little iceball out in the middle of nowhere isn't going to be popping out Elvis anytime soon, so don't bother putting Emphasize Great People in it. For that matter, I wouldn't turn Emphasize Great People on even in your best cities, since assigning the specialists manually gives you greater control over what Great Person is eventually spawns.
Wem said:
I hear talk about specializing cities on commerce and food also.. how is that done? Can / should I specialize a city on research? Is excess food produced by one city shared with food-poor cities, etc?
Food is not shared between cities (unless you count the 1F produced by a Great Merchant super-specialist, but that's kinda a pathetic return for how much it took to make him in the first place).
As many cities as possible should be specialized towards one thing or another. What they specialize in depends on the terrain: lots of hills and a food resource or two means a production city. Lots of flat land and/or coast means a commerce city. Lots of food resources means a Great People city.
How you go about doing the specialization is that you build up the city's buildings and surrounding terrain so as to best suit what it's good at:
-A production city will be surrounded by nothing but hammer-boosters (i.e. Mines) and whatever Farms/Pastures are required to feed those mines...AND NO COTTAGES. Inside the city, there will be Forge/Factory/Power/Barracks/Drydock. However, it won't be bringing in any Commerce, so the science and gold buildings are pretty much useless in their normal function (i.e. the only reason you'd ever put a Market in a production city is for the

boost).
-A Commerce city will be surrounded by Grasslands, Flood Plains, and Coast, with the first 2 mostly covered by Cottages. It actually needs to build its banks and such though, so a couple Mines and/or Watermills are needed, plus whatever farms/pastures are needed to feed everything. Commerce cities' hammer output should be "adequate", and no more. Inside the city, Forges occasionally come in handy, but it's really all about the Bank, Grocer, Market, Monastaries, Library, University, and Observatory. Basically, if it improves the science or gold income of the city, a Commerce city should have it.
-A GP city will be surrounded by food, one way or another. More food = more specialists. More specialists = more GPP. More GPP = more Great People. Thus, Farms/Pastures will be the dominant terrain improvement. Like a commerce city, however, a GP city will need a couple Mines to get its buildings up and running...unless you're running Slavery, in which case you get an absurd amount of people to

into buildings. Most buildings will be for the purpose of either A) letting the city get bigger, or B) allowing you to use more specialists (if not both).
Once you've got the hang of deciding how to specialize based on a city's surrounding terrain, you can start working on choosing the city's specialization first, and THEN building it in suitable terrain for that specialization. In other words, instead of building a bunch of cities and later saying, "hmm, now which one has a lot of hills around it", you look around
before you colonize and say, "hmm, now where's a good spot with lots of hills and some food, making for the perfect production city?"