Dont build:
- farms.
- temples with growth bonus

8% total growth rate is the magic number (will sustain 24000 pop). Use the settlement details scroll and add up the numbers you see.

To maintain public order, you should (in order of being cost effective):
1. Install a governor if you have a spare: you're paying for them anyway!
2. Build temples.
3. Build arenas/execution squares/odeons (public order boosting buildings).
4. Build health buildings and the level 5 market, though these only compensate the problems they generate themselves in terms of squalor, long term.
5. Lower the tax rate. Aim for 85% at high rate, or 75% at low/normal (to avoid the traits efficient taxman & poor assessor).

This should work for most settlements, but for those that are newly conquered and/or far far away:

6. Use as much garrisons as you need if the population is below 16800.
7. Use Games/Races above 16800, where you mix with troops and match. Obviously if you need 25% you can throw Monthly Games (+20%) and use some troops (+5%), you don't need Daily Games (+40%) in that case.