Look for resources just outside your borders and consider purchasing those tiles.
This negates unhappiness from occupation. Use the following methods to increase happiness: There is no magical wellspring of happiness that will allow you to absorb an unlimited number of enemy cities into your empire without repercussions. If those cities are going to drive your happiness into the ground, it may not be in your best interests to take all of them at once. I think your conclusion here may be false. In war the AI sometimes gives up and hands over like 12 cities at once for peace (which I'd be stupid not to take)
