One old standby I (and many others) use is some version of this "Forgotten Favorites":
- Rating - greater than - 2 stars
Sort by - least recently played
... it's a great playlist, but sometimes it gets glutted with mediocre three-star songs (the kind you want to listen to every few songs, not every song) and there aren't any four or five star songs left among your library's 'least recently played.' Thus, the playlist becomes kind of boring.
This can be solved with the new Playlist based on another Playlist variable in v. 4.5. Try the following:
Rating - is - 3 stars
Limit to 20 songs by - least recently played
Rating - is greater than - 3 stars
Limit to 30 songs by - least recently played
Then, the meta-playlist (the one that gets played):
Playlist - is - Playlist A
Playlist - is - Playlist B
Limit to 50 songs by - random
Voila, now you always get 40% 3-star songs, and 60% 4-5 star songs! Adjust numbers to suit personal taste.