My city is close to perfect but I can't seem to get any demand -- it's negative in all categories!
It has a population of 45,000, and all but one bar of mayor rating filled up. EQ (education) is nearly 200 everywhere, all the opinion polls are full (except environment and traffic, which are ~3/4). Average salary ~70,000. Water is supplied to all parts of the city, and the city has full school/library/college/museum/police/fire/hospital coverage (well-funded).
I have no dirty or manufacturing industry, but some high-tech (~3000 jobs); the rest of my city is commercial. I installed a small subway, which is used about half as much as the roads, and I have a LOT of city beautification ($1000/month+ worth of parks, etc.)
I'm pulling in a little over $800 a month with taxes at about 9% for medium/high residential and 7% for commercial/high-tech. There's a few "no jobs" indicators in the residential sectors, but no demand for commerce/industry.
I tried a bunch of different things (without saving), but nothing seems effective. I added an airport and a landmark (Bank of America tower), which temporarily sparked some commercial, but very little. I tried lowering taxes to zero, but even that did nothing except bankrupt me in 6 months.
I'm trying to make a self-sufficient city, so there's no connected cities, but if I must add them I will. I want to explore other options first, though. Thanks!