iTunes 11 has a new Up Next feature that’s sort of in between iTunes DJ and just shuffling your music. You can play music from your library or playlist, add a song to Up Next to make sure you hear it next, and afterwards iTunes continues playing whatever you were original listening to. The Up Next list is also great for seeing what iTunes has planned next when you’re shuffling your library, an album, or a playlist.
To add music to Up Next, you can click the new action arrow next to songs to show a menu of a whole bunch of features, including “Add to Up Next.” A quicker way to add music to Up Next is to hold the option key when browsing. A plus sign will appear next to any song in your library—click it to add the song to Up Next.
Update: In iTunes 11’s new MiniPlayer (⌥-⌘-3 or Window > MiniPlayer) you can also add music to Up Next with these steps, thanks to @chucker on Twitter:
type to search for a song
use the up and down arrows to scroll through the results
press ⌥-Return to add any songs or albums to Up Next