You can choose to manually pick which songs you put on the iPod. Also, if you uncheck the little box next to a song, it won't be transferred to the iPod if you leave it set to sync automatically.
If your collection is a lot bigger than 5GB, there is a utility (I think it may be an AppleScript) that will randomly transfer 5GB-worth of your songs each time you plug in the iPod. The name escapes me at the moment, though. Anyone else know where to find it?