Understanding Ratings and Play/Skip Counts
Keeping your music properly tagged ensures that Genius can recognize it, but there's a lot more that you can do to help ensure that Genius delivers playlists of music that you want to hear.
One of the most important things you can do is to use the iTunes rating function. As you might guess, this is a feature that allows you to rate songs based on how much you like them. The feature, which is enabled using the View Options dialog box (which can be displayed from the View menu), adds a ratings column to the iTunes display.
You can rate songs from one to five stars. This information is included in the information analyzed by Genius (and it is also part of the information that the iTunes servers use to adapt both your results and the results of other people who have similar music collections).
You can also right-click on a song or the iTunes icon of the system tray (Windows) or control-click on the iTunes icon in the Dock (Mac) to rate songs.
If you have a large (or even not-so-large) iTunes library, rating every song can be a time-consuming prospect. Fortunately, iTunes has a couple of other helpful pieces of metadata that it maintains: play counts and skip counts.
As you can probably guess, the play count records how many times you listen to a song in its entirety, and the skip count records how many times you skip over a song while it is playing.
Both sources of information are valuable in terms of identifying your tastes. Genius relies on this data as well as ratings when analyzing your library.
What's important to note about these features is that iPods, iPhones, and Apple TVs all maintain this information as well and will sync it with iTunes whenever they're synced. This means that syncing your devices regularly helps ensure that iTunes has an accurate picture of your musical tastes (as well as delivering the latest Genius results to the device).
If you want to get the advantage of ratings without the effort, there are tools that will attempt to automatically rate the songs in your library based on play and skip counts as well.
This can help serve as a baseline, although you might want to periodically check your ratings (be they manually or automatically created) to both ensure accuracy and to reflect your tastes as they change over time.