It'll depend on a number of factors. Mainly which iPod you're using with it, how much you're willing to spend, charging, line out, etc. Most of them suck, so you'll have your work cut out for you. You'll probably hear alot of "just buy a CD player with an aux in," and that's probably the best option unless you can get a good FM transmitter for real cheap.
So far, the best one I've used, by far is the old tubular Griffin iTrip. Unfortunately, it's only good for 4th gens, minis, and 3rd gens IIRC. It hooks into that little serial port beside the headphone jack that they took out with the 5th gen and nanos. I did an antenna mod (it ended up being 5 or 6 inches long), and it worked almost perfectly. It actually sounded better than most FM stations. No distortion, very little to no static, and I got it for just under $3 shipped off ebay. Unfortunately, it won't work with my iPod Photo while it's in my Yotank case. I tried modding it even further, but the wires are too short, and I'd have to resolder it to be long enough to fit. IIRC, it only works on 87.9 and there's no controls of any kind on it. Just an LED. The quality is pretty astounding for something that old and that cheap, but the biggest drawback is limited compatibility.
Based on your sig, I'm pretty sure that option is out for you, but I thought it might be helpful for others. I've also tried the Griffin iTrip Auto (2007 edition), and it sucks. Not as bad as some others I've used, but it had a good bit of distortion (especially on the low end), gave me really bad feedback when in stereo mode, and the stereo image was pretty messed up. I've also used a couple of Belkin universal ones, and they just plain suck. They'd probably work better if you're out in the desert, otherwise avoid them.
That's what I've been through on my search. Cassette adaptors have worked out much better (except for the iTrip for 4th gen iPods), but it doesn't look like that's an option for you either. I'll eventually get a CD player with an aux in, but I've also learned that I get much more distracted with my iPod in my truck as opposed to using CDs or the radio. I skip around too much when it's on shuffle.