What you need to do is in iTunes when you plug your iPod in you will see a little iPod icon come up in the bottom right corner. Click on it and you will see iPod settings. You will see that your iPod is on automatic update mode. You need to click the bubble that says manual update mode. This means that if you click on a playlist and hit update iPod the entire playlist but only the playlist will be transfered and all existing playlists will remain the same.
VERY IMPORTANT: On manual mode you need to click the little eject icon next to your iPod icon and name that appears in the playlist window on the far left. If you do not you will get a do not disconnect error.
I hope this helps.
P.S. I've done this on three of our computers and I can use my ipod on each and choose only the playlists I want while keeping the others.
Somedays you're the dog,
somedays you're the hydrant.