Well, I guess the Actual question would be "How the save?" but that's terrible grammar.
In my group, we have a house-rule that makes the use of Zone of Truth a bit more... useful, for lack of a better word.
The rule is as follows: Successfully making the save against Zone of Truth does not allow the person who made the save to ignore the effects; it only makes them aware of the effect, thus allowing them to refuse to answer or to phrase their sentences less directly to avoid deliberate false-hoods while still not speaking clearly. If the save is failed, then the person finds her/himself unable to prevent her/himself from speaking anything but the most direct truth possible.
My question is: For people that fail the save, what is a good way to depict them not realizing they're being forced to speak the truth? It would be obvious, of course, but the main issue is the first few sentences, when they've yet to become aware of it. Does anyone else use a similar rule, or have any experience with a similar situation?