I find it difficult to control regions as well, though there are some tricks you can utilise in order to prevent using large garissons:
1) Build temples that give law bonuses
2) Don't build any population growing buildings, unless you get some stable public order
3) Try and keep your capital in the center of your empire, or just start conquering regions in the perimeter of your starting one (if you don't want to change it)
4) Make population ''exchanges'', by using the add_population cheat. For example, you capture a settlement and decide that, even after you've expelled them, they are still too numerous. Just use ''add_population X -5000'' for example and then ''add_population Y 5000'', where X represents the captured city and Y the city where the population is going to move.

Hope this helped,
Maion