One turn before you DOW on an AI civ you hate, concentrate your force near this CS, trade this CS to this AI leader. Get all the gold, relics and any useful great works in the deal and don't bother the gpt or luxury. Then declare war, pillage everything, take back the CS and liberate it.
You gain all gold of your enemy and prevent the AI from gold-purchasing units, may get some relics and great works, get more from pillaging, gain the suzerain status of a CS, and the +5 dip rating for liberating a city. I actually consider this one of the biggest exploits in this game, and limit myself using only once per game.