click the little boxes with checkmarks beside the song(s) you dont want. after clicking it should be a plain box with nothing inside. if that doesnt work plug in your ipod. click on it when it comes up on your computer and click where it says 'sync only song that are checked' or something like that.

You can set your iTunes to "Manually manage music and video" at first. And then uncheck the songs your don't want to sync to iPod on the playlist, done. Then iTunes will not transfer the songs to your iPod when you sync iPod.
For more Tips about iPod, you may refer to this iPod FAQ.
http://www.mac-dvd.com/ipod-tips-and-ipod-faq.html
Hope it helps. Contact me if you have any other question.