If you go to city details screen, and hover the mouse over the Unrest stats, you will see a tooltip with all factors modifying it. There are some of them, so to be brief, and for this case, the most important ones, are:
- Conquered cities recieve a 40% unrest penalty, that will decrease over time (maybe around 1% per turn, so make the account...). That's what you are mainly dealing with now. It takes long to make a city operative after conquering it.
- City dominion not linked to your own dominion causes around a 15% unrest penalty. You can solve it by building outposts and expand the Zone of Control to link it.
- City buildings. Some of them give a bonus against unrest (bell tower, cleric...). But if you have to build them you will have to wait a lot, so rushing the building is the solution (wars are said to be expensive...). Start by rushing the most effective (cleric if more than 2 essence, bell tower if not), maybe rush one more next turn.
- Champions with Path of the Governor. Probably the only use for these guys, but I have to say it is very effective. The more traits as Administrator (I,II,III...) the more effective. Who has a champ with POTG? Well, you will have from now if you pretend to be a conqueror 
If you play altar, is easy to use one or two of your henchmen to become that.
- Taxes. Empire taxes causes a general level of unrest. Maybe reducing them could help, if no other option available, and you can sacrifice gildars for that. But in general it doesn't deserve.
Probably, there are other ways to reduce unrest, but they are very situational. For example, there is a spell to reduce unrest in a city, but you need to have it learnt, and it takes a huge amount of mana, so I don't count with it. And there is a city upgrade that reduces unrest, but again, situational: it is a high level city upgrade, and you will use it only for these moments.
Finally, the ultimate way in unrest-control tactics: raze the ****** city! (after a 5 turns cooldown, don't ask me why)
