Use forts - identical to cities when it comes to defections (they won't ever defect).

Whenever I capture Italy I have a small army, forted, a bit west of Bologna. The cities only have as much garrison as they can support for free. Keeps control of the entire region nicely by being able to reach every city in 1 turn, reinforcing in case the AI wants to assault. (Or I get bored and just sally out and kill them - with the broken AI not doing anything)