You can also use the iTunes widget in the "Today" view of the notifications sidebar.
I've got my notifications sidebar set to show when I move the cursor up to the upper-right corner of my screen, so to control itunes I just have to swipe to bring that into view. You can set it however you like in system preferences, although the settings are scattered - you'll find the "hot corners" under the desktop and screensaver settings, or you can use the mouse or trackpad settings to set other triggers. If you have a Magic Trackpad, I think the default setting is to show notifications if you swipe with two fingers from the right edge of the trackpad.
Plus there's lots of add-on software that gives you little remote controls for itunes that might live in your menubar, or dock, etc.
And don't forget that you have dedicated keys in the top row of your keyboard (assuming you have an Apple keyboard) to skip forward, back, play/pause, volume up/down etc - those work all the time, no matter what app you're currently working in.