I've got a system that works great for me... most importantly, it keeps my music collection fresh. I've done some creative (IMHO) stuff with grouping & rating that makes it all work right.
First off, my ratings system:
5 star: a favorite song, welcome in multi-genre mixes ("crossover")
4 star: very good song, welcome in multi-genre mixes
3 star: good song if I'm listening to this genre, but doesn't rank as "crossover"
2 star: uncheck next time you sync... i.e. keep in iTunes, remove from iPod
1 star: yuck. delete this next time you sync.
0 star: unrated
I add identifying tags to tracks' "Grouping" fields, separated by a space. For example, a given track might have the grouping "Quiet NoShuffle" or "Christian Alternative." My grouping tags:
"NoShuffle" - never put these in a cross-genre mix. Comedy, spoken word, video, podcasts, classical, etc.
"Country" - I use this to group genres like Country, New Bluegrass, Alt. Country, etc.
"Christian" - groups genres like Worship, Christian Rock, Christian Pop, etc.
"Alternative" - groups genres like Adult Alt., Alt/Indy Rock, Electronic, Alt. Country, etc.
"Quiet" - music quiet enough to go to sleep to. For me, that's mostly New Age piano & some classical.
These ratings and tags feed my smart playlists. My heavy rotation playlists all start with a "." (period) so they're sorted to the top. These are all "Match All of the following rules" playlists. Here are some examples of my playlists:
.4+LP All LOP: Means "4 stars or better, low play, all genres, least often played"
- Grouping does not contain "NoShuffle"
- My Rating is greater than 3
- Last played is not in the last 2 days
- Limit to 300 items selected by least often played
.4+LP All LRP: Means "4 stars+, low play, all genres, least recently played"
- Grouping does not contain "NoShuffle"
- My Rating is greater than 3
- Limit to 300 items selected by least recently played
.4+LP No Country: to play for country-hating friends Also have a .4+LP No Christian.
- Grouping does not contain "Country"
- Grouping does not contain "NoShuffle"
- My Rating is greater than 3
- Limit to 100 items selected by least often played
.Quiet (must allow 0 star songs, as most of my instrumentals are unrated)
- Grouping contains "Quiet"
- My rating is not 1
- My rating is not 2
- Limit to 95 items selected by least often played
3+LP Country&Bluegrass (have similar "3+LP"s for Alternative, Christian, Jazz...)
- Grouping contains "Country"
- My Rating is greater than 2
- Last played is not in the last 2 days
- Limit to 200 items selected by least often played
Unrated All (also have Unrated Country, Unrated Christian, Unrated Jazz, etc.)
- My Rating is 0
- Grouping does not contain "NoShuffle"
- Kind does not contain "video" (don't need in other PLs, as my videos are all unrated)
I've found selecting a certain number of songs by least often played works better than saying "not played within the past 2 weeks." The latter method doesn't favor the song you last played 6 months ago over the song you last played 15 days ago, so your collection's more likely to get stagnant.
I use my .4+LP & 3+LP lists for driving, biking, etc., while my Unrated playlists get played mostly on train rides, etc. when I have eyes & hands free.