If you want to get a little more high tech then just plugging into aux inputs/tape adapter/fm modulator then there are a couple companies that make units that plug into the dock connector on your ipod (compatible with any of the ipods that have dock connectors I think - 4th gen, mini, nano, video, photo) and then plugs into the cd changer port on the back of your car deck. This obviously only works if you have a deck with a cd changer port, but pretty much ever aftermarket deck sold these days has one, and it also can be used on most factory stereos, as a cd changer has been an option for quite some time now. Not only does this unit give a true audio connection (perfectly clear, unlike say an fm modulator), but it also charges the ipod, displays the song information stored on the ID tag of the music file, and, best of all, allows you to control the ipod through the controls on the car deck. This means you can tuck the ipod away in the glove box or something and power up/down and control the ipod with the deck. Obviously different decks have different levels of compatibility with these various features, but the Dension website has a compatibility menu to see what functions your factory or aftermarket deck will support. If you have not bought a deck yet, you can also buy ipod compatible ones, that, by the name, suggest that they are fully compatible with all the features of the ipod, but I have not really looked into those too much. Anyways, check out the website, it is a pretty cool feature, but for now I just stick with aux inputs on the back of my deck.
unit is call ice>link plus, here is the website:
http://www.densionusa.com/icelink_info.htm
Brad