I want to use an NFC tag with my HTC One in the car, obviously to enable things like bluetooth, start up satnav etc.
However, my question is how does one put it in another state when leaving the car. Is it a case of swiping again, or do I have a second tag. I don't really like the idea of it toggling as I will never be quite sure which state I am in.What's the normal thing to do here ?

Try an app called NFC Task Launcher.

With that you can program tags that toggle, or you can program two separate tags. Whichever you prefer.