I use an iTrip which is fine if the FM frequency I'm tuned to is "free" for my whole journey. So it's ok around home and for short jaunts. On longer journeys I get interference on the FM frequency and so have to retune both iPod (Nano) and car radio. There is alos the problem of running the iPod batteries down.

On Easyipod.co.uk they have a system which is plumbed into the car radio system, like a proper mobile phone charger / holder user. This gets rid of the wires from one of the cassette type players and doesn't need a lead from the cigarette lighter to recharge it. It's a bit pricey though but I might go for this option in the long run.

I'll be interested to see what other ideas this thread brings up.