I usually switch apps using the "double-tap the home button to access the app switcher" shortcut, but recently learned that there's an easier way. Excuse me if you already know this one, but it was new to me (a few months ago) and most of the people I've shown it to have been surprised.

I made a short (42 second) video that shows both techniques—the old school double-tap the home button technique first, followed by the newfangled 3D press/swipe on the left edge of the screen technique.

Enjoy!