i've only been playing on warlord so far, i'm too chicken to try regent but im gaining courage (it'll be my next game), but from what i have seen, culture flipping happens mostly on boarder cities, if it is directly on the boarder of the civ you are attacking, thus a tactic i use if i think i may have trouble invading swiftly is to destroy the next few cities i capture to make a sort of culture gap. I dont know if this works and if i've just been lucky, but it seems to help me. Also it might just be because i play on a low level.
And another addition would be if cities had sort of "civil wars" this would occur on a flip if a city was split between 2 nationalities. It would cause the population to drop and the citizens would fight just like normal units, you wouldnt' controll it though, which ever nationality wins would decide who gets the city, but the population would be decreaced and many of the enhancements would be destroyed if it was kept by the original civ, all if it changed hands. secondly this would prevent the city from ever being captured, if it was invaded, the next time it would have to be raised, there would be no option of it switching, as this city would have an increase in patriotism and therefore would fight to the death to avoid allowing the city fall into enemy hands, and thus it would have no more culture flips.
That idea would happen randomly of course, not too often but say 1 out of every 5 flips it would occur.