A few general things:
Walls: Try to avoid square shapes and excess corner pieces. Very few cities were built to make a specific geometric shape; rather walls were erected as the city became important and thus vulnerable to attack. And excess corner pieces look wrong.
Roads: Mix your road types, and use dirt1 in places also. The nice, shiny road terrain should be used sparingly; broken road should be your main road type. It's more accurate.
Lay out the important areas first: the castle, the docks, the central markets, etc. Then fill the rest with extra markets/blacksmiths/etc. and houses. In the crowded sections where a lot of people live use lots of close-together houses, map copying if needed to fill up space, but be sure that most of the buildings can be "entered" from the street. It's more realistic (think about it...).
Mix civilizations and ages when it comes to building types. You'd be surprised what works. Especially combined with map copy.
Avoid using large single buildings for important stuff, e.g. a castle for your main castle. Use map copy to make uberbuildings - if you do it right, it looks much better. Alternatively, use a single large building and place details in the surrounding area (like a courtyard). Alternatively, do both.
Add people.
Add more people.
It's a city, man! ADD PEOPLE!!!
Use triggers to task/patrol people.
Mix terrain where there's no roads. Even in a city, areas of pure grass1 still look boring.
Test crowded areas to make sure you can walk through easily. Players hate having to take the long way.
Build 'Taverns' amidst your crowds of houses for variety. Markets and Blacksmiths are nice, but a custom-built Tavern with map copy is much more exciting.
Rename stuff. It's more fun to explore a city with 'unique' objects than boring old 'Markets' and 'Blacksmiths.'
That should be plenty for now...
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