I've found that the more automated or generic the Smart Playlist, the less it reflects my music tastes.
I have several nested smart playlists that I use to keep a fresh choice of music on my iPod. They're arranged like this:
"Master List" contains all the songs I want to have available to my nested lists. This way I can easily eliminate anything from my mixes, such as certain artists or genres.
"5 Stars" contains all my 5 star songs that I haven't heard in the last week.
"4 Stars" contains all my 4 star songs that I haven't heard in the last two weeks.
and so on, down to
"0 Stars Recent" contains all my unrated songs that I added in the last month and haven't heard in a week.
"0 Stars Older" contains all my unrated songs that I haven't heard in the last two months.
Then I have a shuffle list, which takes all the songs from each playlist, and mixes them together. If you want to weight how many of each song is in the list, simply limit the "_ Stars" playlists to a certain amount of songs or Mb.
My shuffle list is the second smart playlist in my "sync" folder. The first smart playlist is linked to a regular playlist that I add all the songs I want to keep on my iPod for a while. I like to put my newest albums into this playlist so that they're guaranteed to be on my iPod.
Finally, I have one more playlist called "iPod" that contains everything in the "Sync" folder. This is only because I can't set my iPod to sync with a folder; only with a playlist. That means that in this "iPod" playlist, the order contains all the songs I want permanently on my iPod, and the remainder is a mix of stuff I haven't heard in a while.