The iPod and iPhone gives you a choice: You can either have the device sync all your music automatically by playlist, or you can select "Manually manage music and videos," which lets you click-and-drag songs on to the Pod.
The trouble is, you can't do both. Say you want the Pod to automatically synch smart playlists -- maybe playlists of recent albums and recent podcasts and four-star songs -- but you *also* want to be able to drag songs onto the pod? Tough, Apple won't let you: If you select "manually manage," the playlists won't sync, and if you unselect it, the clicking-and-dragging won't work.
Here's the workaround:
1) If "manually manage music" is on, turn it off -- that is, uncheck it (you'll see this option on the screen that comes up when you plug in your pod).
2) Create a playlist for arbitrarily adding songs -- call it whatever you like.
3) In the music tab, sync the playlist you just created -- along with any others you want automatically updated.
4) Now you can have both worlds -- smart playlists (recent albums, top songs, etc) automatically go to the Pod, and whenever you want some other song to take on the road, just drag it in to the iPhone/iPod playlist you just created.