A quick shortcut to check and uncheck all items in your playlists
If you have a lot of items in a music playlist and you want to check or uncheck them all, you can save yourself the time and trouble of doing it one-by-one simply by clicking the checkbox of any song in the playlist while holding the Command key down. Doing so will check or uncheck all the items in the playlist at once.
By default, iTunes displays the approximate playing time of the selected playlist in the bottom status bar – along with the number of songs and file size of the list of songs (See image below). 
This is fine most of the time. But what about when you’re trying to find the perfect combination of songs to use as a soundtrack for your home movie, etc.? How long exactly is 5.9 hours? 
Click the info in the status bar with your mouse and the status display switches from relative to exact time (as seen above).