Compared to civII the trade system is marvelous. For more interesting and realitic. In just regret two or three thinks :
1) We cannot trade food.
2) we cannot trade production.
3) we cannot get loan from a bank (or other civ ...).

I was first some complicate to understand the trade system, but now I begin to be a good merchand.

I just ended a long campeign in big desert a the frontier of my "Metropol" (with the french Civilisation). Just to take control on petrol ressouces and to have a nevralgic center my trade line that allow goods to travel safty from my colony to my capital territory.

Generaly for one kind of product, I establish a city who will be the center for this stuff. I generaly choose a city which have already one or more square with this ressource in its vinicy. I boost its economy with a market place and a bank... Income become very interesting :
On a my city (Pompei) was the center of "tabacs" with 3 squares of tabacs in its vinicy and 4 city giving its tabacs. The base income was :
10+20+30+40+50+60+70 = 280 gold / turn.

Have you another trade strategy ?

IA are poor merchand. There is only few and uninteresting trade relationships with an IA.
Is there some mods which give the more intelligence for trading ?