Stop using them. Mount candles on the front of your car instead.
Seriously, though, HIDs last like 2000-3000 hours or so. Just turn them on when you need them and turn them off when you don't. When they burn out, buy new ones.
edit: The only thing I've ever heard, was that turning them on/off multiple times over a short period
could effect life, but that was from a single source on the internet, that I can't give any credence to.