ah right, ok.
smart playlists are the purple ones, normal playlists are the blue ones in the left hand side of iTunes. choose file > new smart playlist. it brings up a dialogue box with a list of options. these are perameters for this smart folder. you can make smart playlists for pretty much any use, and is especially useful for this. put the settings that i have shown you into a new smart playlist. now, with the ipod attatched, click on the ipod button in the bottom right corner of itunes (or the ipod tab in the itunes preferences). choose the second option for you would like iTunes to fill the ipod, which is Automatically update selected playlists: followed by the list of all your plalists in iTunes.