Here are some keyboard shortcuts to let you switch between the mini player and the full view in iTunes 11. You can also opt to keep both views open at the same time.
1. Open the iTunes application.
2. Use this command to switch between regular view and the mini player:
Command + Option + M 3.
Use this command to open the mini player while also keeping the full view open:
Command + Option + 3
For more, see the original article at the link below.
The Ultimate List Of iTunes 11 Tips, Tricks And Changes | Cult of Mac