Of course, if money's no object then all the improvements that boost population are a must - but I guess you're already doing that. Low taxes, farms and sanitation all work well together. If money is short, then things get more tricky. A lot of high-trade cash cow cities can go a long way towards supporting the growth of you capital and couple of other home cities.

If your city has a governor, a good way to enhance city growth is to embark on a campaign of vigorous conquest. Make sure you enslave the population of every city you come across, and you'll boost the population in governed cities. Of course, the problem is that this hurts the growth of these conquered cities later on - but maybe you don't care about the barbarians.