Population problems

[Cambyses]

Take a look in your cities and see what is causing the unrest.

If the worst offender is "distance from capital" then move your capital
If its cultural, then booming populations is what you need, as building your own markets, temples and especially governor's buildings has a radical effect on cultural unrest
If its a high amount of generic unrest it could be an enemy spy
Maybe your taxes are too high
If its just massive squalor then you need health buildings

In the short term geting a decent governor in, recruiting lots of large units of troops and filling the build queue with +happiness buildings is probably the answer. Cities can riot but wont rebel if you have enough queued +happiness buildings that predict you a happiness score of 70 or more. Longer term you really need to get to the root cause of the problem though.

In my experience once cultural unrest has been taken care of its extremely unusual (although still possible unfortunately) to have further happiness problems.