The art of quitting iPhone apps
The addition of multitasking to the Apple iOS 4 operating system has left many users — including me — with quickly draining batteries and a need to learn how to quit iPhone applications. This was never an issue in previous versions of the iOS, as programs automatically shut down when the user exited and opened a different app.
I've gotten into the habit of checking to see which apps are waiting in the background, and quitting those that drain the battery. These include most apps that use the iPhone's global positioning system, such as Trapster, an app that I'll feature in my Sunday column. Trapster CEO Pete Tenereillo said teaching iPhone users how to quit apps has become an important function for his company since the introduction of iOS 4.
Quitting apps in iOS 4
1. Press the home button (the only button on the front of the iPhone).
2. Tap "close" (in Trapster) if prompted. 
3. Double-tap the home button, which will reveal icons at the bottom of the screen of the apps running in the background and those recently used.
4. Press and hold your finger on the icon of the app you want to quit. This will result in a minus sign (-) in a red circle at the top-left corner of the icons. The icons will quiver, just as they do when you reposition them into folders or on the home screens.
5. Tap the minus sign of the app you are quitting. 
6. Double-tap the home button to return to the iPhone home screen.
For more information on app-quitting in iOS 4 see this entry at The Unofficial Apple Weblog:
http://bit.ly/tuawappkill
