My active iTunes library is 6,800 tracks long, and for the purposes of reviews, I go through a hundred or more new songs each month. This amount of music would be daunting to manage, but I have picked up a few habits over the years that helps me keep it all in order:

When adding songs to the library, I always rate them as one star.
I have a smart play-list that only shows songs that have one star and have a play-count of 0.
This allows me to quickly go over the new songs that I have yet to hear. I only listen to this play-list when I have the time and presence of mind to focus on the song. I then rate the songs from that starting point as follows:

A decent song gets 2 stars.
Good songs get 3 stars.
Fantastic songs get 4
Songs that are beyond compare get the coveted 5 star rating. (in the library of almost 7,000 songs there are only 57 that have that rating.
If the song fails the test, and is lacking in some way; I remove its one star and rate it 0. This flags the song for either deletion or repair.
There really isn’t a place for songs of one star for long, they tend to cycle through my play-list until they get rated up or down, I then use the sort by rating option to find and remove/fix album art/lyrics on the 0 star tracks.