For my bday I got a nifty new deck that came with an ipod install - the install guys ran the connector cord thingy (i'm such a girl) into the glove box and i just plug my ipod into that when i'm in my car. I have direct access to the entire ipod via the deck - its actually easier to use than the ipod's controls (go figure.)
Before that, I just used an iTrip (by Griffin) and when not in the car took the ipod with me. It sends the ipod signal via radio to your car radio. A little staticy at times but it got the job done.