I've been fiddling with the iTunes smart playlist feature for awhile now and I thought I'd see what others have come up with for playlist ideas. Here's one I use all the time:
"Stale Favorites"
Match all conditions
Rating is greater than 3 stars
Last played in not in the last 2 weeks
My iPod is only 2GB so I organize it using three smart playlists:
"iPod Classical"
Match all conditions
Genre contains Classical
Last played in not in the last 10 days
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Limit to 450MB selected by Album
"iPod Favorites" (references an outside favorite songs list that's way to big to fit)
Match all conditions
Playlist is "Favorites"
Last played is not in the last 10 days
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Limit to 850MB selected at random
"iPod Jazz/Blues"
Match any condition
Genre contains "Jazz"
Genre contains "Blues"
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Limit to 350MB selected by least recently played
Lets hear some others.