Is your iPhone acting weird, are calls dropping, maybe your applications are freezing? There are a few tips that you can try below to see if you might be able restore its functionally back to normal.
Killing A Frozen Applications
iOS 3: If you're in a application that stops working, you can kill that application by holding down the Home button for about 6 seconds. The application should close and you should be returned to the Home screen.
iOS 4: Press the Home button once to return to the Home screen. Then double press the Home button to pull up the application switcher. In the application switcher, touch and hold the icon of the frozen application for about two second. When the icons starts vibrating, press the minus icon with a circle in it to kill that application.
Sometimes it handy to be able to grab a screen shot of an image on the display because it could be showing a problem you're having with the application or the iOS. By holding down both the Wake/Power and Home buttons for about 1 seconds, the screen will flash and you will hear a camera click. A screen shot of the image on the display will be saved to the photo roll under the Picture application.
Turning off the phone
By holding down both the Wake/Power and Home buttons for about 5 seconds, will display a slider will that says "slide to power off". If you slide the arrow to the right it will cause the phone to shutdown. To turn the phone back on, press the Wake/Power for a few seconds, then you should see the Apple logo display on the screen.
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Killing A Frozen Applications
iOS 3: If you're in a application that stops working, you can kill that application by holding down the Home button for about 6 seconds. The application should close and you should be returned to the Home screen.
iOS 4: Press the Home button once to return to the Home screen. Then double press the Home button to pull up the application switcher. In the application switcher, touch and hold the icon of the frozen application for about two second. When the icons starts vibrating, press the minus icon with a circle in it to kill that application.
Note: This is a great trick to use when applications (like Mail or the Safari browser) are not able to connect to the Internet. In my experience, when I killed the program using the application switcher and restarted it, generally it started working correctly again.Taking a Screen Shot
Sometimes it handy to be able to grab a screen shot of an image on the display because it could be showing a problem you're having with the application or the iOS. By holding down both the Wake/Power and Home buttons for about 1 seconds, the screen will flash and you will hear a camera click. A screen shot of the image on the display will be saved to the photo roll under the Picture application.
Turning off the phone
By holding down both the Wake/Power and Home buttons for about 5 seconds, will display a slider will that says "slide to power off". If you slide the arrow to the right it will cause the phone to shutdown. To turn the phone back on, press the Wake/Power for a few seconds, then you should see the Apple logo display on the screen.