Short and sweet, never EVER EVER put an army into a foreign civ's city after 5 turns of capture, because after that it (in my experience) the city is much more likely to flip. If my army gets damaged, i usually stack it with another army in the field and heal, i know it takes longer, but then i havent lost 20 turns of production to a culture flip. I wait untill my 1st wave is stalled, sending in my second wave to finish them, then when they are finished, i move in to the barracks cities and heal them, ready for my next conquest of a poor 3rd world country.