the way i set up my smart playlists is by using a kind of "quota" system. i'm quite genre orientated, i have a 15gb pod, so i tend to set up quotas of particular genres, say a random 3gigs of rock, 3gigs of live stuff etc. then i make another list with a random selection of whatever's left. using checkboxes i can disclude tracks that i don't want to listen to on the move or am bored of.
this is quicker than making stacks of "manual" playlists... seems to work for me