In a perfect situation, I garrison the town with enough troops to end the resistance in a turn or two, then rush out workers/settlers until the pop is down to 1. Then it gets sticky. In the original version of CivIII, if you built those workers back into the town, they'd be your nationality. I don't know if that works on later versions or not (just bought C3Complete, haven't gotten that far in a game yet, I'll let you know.)
Another good tactic, if you have the transport capacity, is to bring along a buttload of workers when you take the town and build them in immediately. If neither of these options are available to me (for lack of time or transport capacity or whatever), I put a match to it. If I can't have the Pyramids, no one can.